Fungi in compost

When planting up seeds in small container using a bought general all purpose compost, with clear raised lids for propagating. I noticed what looked like white dots had appeared on the surface over night. Out with the magnifying glass, I was amazed to see they were tiny what looked like mushrooms. By the time I had got my camera, they had shrivelled up. So next morning, camera ready I tried to photograph them. These photos were taken on Macro, then the photo cut down and enlarged.

Size: slightly larger than a pin head.

Colour: white, but some that had not fully developed into mushroom shape had pale green in the middle of cap.

Gills: too small to see.

Grow overnight, but shrivel up with in minutes of the lid being taken off seed tray.

I am intrigued to know what they are as I for one have never seen a fungi so small.