This delicate, tiny mushroom growing from Sphagnum moss is a species of Omphalina, possibly in the Omphalina pyxidata complex. Omphalina is a Basidiomycota, one of five divisions in the Kingdom Fungi. Basidiomycota is the fungus division that includes the white button mushroom, shiitake, boletes, and bracket fungi, among many others.
The Sphagnum moss from which the mushroom is sprouting was growing in an unusual place. It was in a shaded spot in a forest on a north-facing vertical rock face with water seeping from cracks.
On some of the moss plants there is bumpy green growth that may be an algae (Coccomyxa?). Is this Omphalina mushroom part of a basidiolichen and not a mere saprophyte?
Basidiolichens are a lichen symbiosis composed of a green alga (sometimes with a cyanobacterium) and a Basidiomycota fungus. Most lichens are formed by fungi in the division Ascomycota.

The genus Omphalina has undergone taxonomic revisions lately and has been split into other genera. One of these is Lichenomphalina, a basidiolichen. Maybe the mushrooms in the photos are a Lichenomphalina, but until I go back to the site where I found it, I won’t know for certain.
More reading
- New Insights into Lichenization in Agaricomycetes Based on an Unusual New Basidiolichen Species of Omphalina s. str.
- Circumscription and typification of sphagnicolous omphalinoid species of Arrhenia (Hygrophoraceae) in Newfoundland and Labrador: three obligate and one facultative species
- Genomic analysis of Coccomyxa viridis, a common low-abundance alga associated with lichen symbioses
