A small, fragile species that can be abundant in mossy turf.  The cap is white, with a yellowish or creamy buff centre and a striate, toothed edge.  It is 1 to 2 cm across  when mature. It is conical when young becoming more bell shaped and flattened with a slight umbo later. The gills are whitish, adnexed or emarginate, and often with a short decurrent tooth. 

several other small white fungi can occur in grasslands.  

Photograph from top down, in side view and from underneath to show gills and full length of stipe.  Note habitat and substrate.

grassland

Autumn.

Fairly common and widespread in Britain.

Fairly frequent in Leicestershire and Rutland.

https://www.first-nature.com/fungi/atheniella-flavoalba.php