I think I'm gonna go along with the "silt" comment too.
I have something similar to this that I have to wipe off every evening. Do your powerheads hit the rock at all, or in the sand? With all that life in your tank that you mentioned, it is possible that sand is getting stirred up and stuff that was on your rock (regular dirt, that might lay if it isn't getting waterflow) will be propelled into your water column, and sometimes collect on the algae already on your glass. My tank will do this badly after I take the turkey baster to it, or something of the like.
That's my guess. Does it just look like...dust?