All sections of text in italics are quotations, sources given.
I found this rare lichen growing on/in mosses on the very damp oceanic microclimate of the Ardingly Sandstone outcrops at Eridge Rocks. Eridge Rocks | Sussex Wildlife Trust. I had previous found what I thought was Lichenomphalia umbellifera, also growing in moss, at Lake Wood, Uckfield (the same geological sequence and microclimate as at Eridge Rocks); however, with that example there was no clear "pea"-shaped thallus, which houses the photobiont (in this lichen's case, a Coccomyxa sp. alga) which meant it did not meet the morphological criteria to be identified as L. umbellifera. This Eridge Rocks example has been verified as Lichenomphalia umbellifera. It has recently been suggested that a more appropriate name is Lichenomphalia ericetorum see: Andrus Voitk, Greg Thorn & Irja Saar Lichenomphalia umbellifera: fungible and infungible epithets and species concepts (2022) 137-4 629voitk22-032.indd (mycotaxon.com) 
Lichens are stable mutualist symbioses of fungi (the mycobiont) and algae or cyanobacteria (the photobiont, which photosynthesises energy for the mycobiont). The fungi that form the mycobionts are nearly all Ascomycota, aka sac fungi, a phylum of fungi (kingdom Fungi) characterized by a saclike structure, the ascus, which contains four to eight ascospores in the sexual stage. Ascomycota | Description, Fungi, Examples, & Facts | Britannica
Nearly all lichens belong to the Ascomycota, but there are some lichenized basidiomycetes (often called “basidiolichens”)… and a few of those actually form mushrooms! Lichenomphalia umbellifera is a lichen that forms a small agaric. [An agaric is a type of fungus fruiting body characterized by the presence of a pileus (cap) that is clearly differentiated from the stipe (stalk)] Agaric - Wikipedia], ...If you find this mushroom, you would probably dismiss it as just another LBM [little brown mushroom], unless you notice the lichen at its base. The lichen part of this mushroom consists of tiny green bubbles that seem to be sprinkled over its substrate. #230: Lichenomphalia umbellifera – Fungus Fact Friday. What is referred to here is this lichen's green thallus
Lichenomphalia umbellifera, also known as the lichen agaric or the green-pea mushroom lichen, is a species of basidiolichen in the family Hygrophoraceae. L. umbellifera forms a symbiotic relationship with unicellular algae in the genus Coccomyxa. The mushroom is white to yellowish-tan and hygrophanous, and occurs throughout most of the year on damp soil and rotting wood. It can be found in the Northern Hemisphere, particularly in the region of the Arctic. Heath Navel (Lichenomphalia umbellifera) · iNaturalist
As the mushroom grows, the cap develops yellowish tones and becomes depressed in the center. Sometimes the central depression becomes very deep and funnel-shaped. The margin unfurls with age and usually becomes pleated or scalloped, making the cap look slightly veined. Underneath the cap, the gills are pale yellowish, decurrent, and widely spaced from each other. The gills sometimes fork or form veins linking adjacent gills. The stipe attaches to the center of the pileus and is reddish-brown at the tip. Toward the center of the stipe, the colors fade to yellow-brown or light yellowish and the stipe often curves. The base of the stipe is usually slightly enlarged and covered with pale fuzz. 
Young L. umbellifera mushrooms have an inrolled margin and may not have a depression in the center. At any age, the gills are decurrent and the stipe is darkest at the top. ;;
The lichen part of the fungus (which is the main body of the fungus) consists of small green globs. The globs appear dusted over the surface of the substrate like a weird kind of powdered sugar. Microscopically, the lichen has an outer layer of clear hyphae followed by a layer of gelatinous material and finally a clump of algal cells in the middle. Hyphae also crisscross the interior of the glob. This is very different than the well-organized structures formed by ascomycotan lichens.
Ecology
You can find L. umbellifera fruiting on the ground, in moss, or on fallen conifer logs in the summer (or possibly in other seasons in warmer climates). Of course, the lichen part is present all year round, but it’s much easier to identify when there is a mushroom growing from it. L. umbellifera grows on every continent except Antarctica, but is rare in most places. ...
To form a lichen, L. umbellifera associates with green algae in the genus Coccomyxa. Coccomyxa species also form lichens with Ascomycota species. Because of this, I suspect that Lichenomphalia evolved its lichen ecology by copying ascomycotan lichens (I couldn’t find any scientific studies looking at how L. umbellifera became lichenized, so you’ll have to make do with my speculation). There are ascomycotan lichens that grow on the ground, so a Lichenomphalia ancestor could have evolved to grow hyphae into an ascomycotan lichen and steal the sugars produced by the algae – essentially parasitizing the ascomycotan lichen. Over time, it could have evolved the cellular machinery to duplicate what the ascomycotan lichen was doing and eventually was able to create a suitable home for Coccomyxa without needing the ascomycotan lichen. At this point it ceased to be a parasite and became a full lichen. Of course, this is mere speculation and would be difficult – if not impossible – to prove. Still, researching how basidiomycete mushrooms evolved into lichens would be fascinating because of all the weird relationships involved. #230: Lichenomphalia umbellifera – Fungus Fact Friday