What happened to my dwarf hairgrass

[jcmcn5]

It looks like two patched of dwarf hairgrass have turned into some kind of solid moss. Pix attached. Has anyone had this happen? Any insights would be appreciated

 

[Autumnsky]

Looks like algae. It seems to be right under the light. You might try to reduce the light timing. Or you could add a little screen piece right above that spot on the glass to reduce the amount of light. If you don't have window screen, a piece of plastic needle point canvas is easy to cut and cheap, from a local craft store. It just blocks some of the light.