Not a clue

[Eck36]

I had to blackout my 29g to beat back the green hair algae that showed up one day, when I took the cover off my tank, this was laying in the sand.

 

[frisbee]

Operculum.

 

[DianaKay]

I think it is what snail leave when they die
I hope someone can confirm or tell us. I have had a few also.

 

[nanoreefing4fun]

Well done frisbee ! when I googled
Operculum.

hit this...

http://en.wikipedia.org/wiki/Operculum_(gastropod)

 

[PSU4ME]

Sure is...... Nice ID!