+1 Erin,they may even be tiny little sponges starting to growIf it's on the glass, it's not ich.
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+1 BJ
And I can't believe this got 2 page 2 b4 u guys figured that one out!!! XD
(I knew it as soon as I read his post =P)
They are in pretty much every tank, and they like to form on the undersides of rocks and such, also on the back tank glass and in your sump...
Just keep them off your clean sides of the glass with regular scrapings & cleaning with a magfloat.
They are good guys, harmless!
Edit: (Oh I guess Sen did have it XP)