Looks like diatoms to me.
Addition: and the start of a growth of hair algae on top of it.
Based on what you have listed in your sig, I'd say more janitors, particularly snails.
http://www.garf.org/algae2/janitor.html is a great source of information.
I've seen some posts recently that may provide you with alternative/better sources if you don't like the idea of a package/per gallons.