OK, I thought it was this:
Do you have a filter that uses that white floss wool stuff - the first 'mechanical' filter stage usually. I thought it was just loose bits of white fluff from this sponge gradually attaching to the surface of your tank glass getting blown there by the flow of water from the filter - it happens with me, but only because (I think) I have an external Eheim that outputs a good stong flow with good direction and therefore 'concentrates' the build up of fluff on the tank wall opposite.
If I'm in my lazy mode (99% of the time then), if I don't clean it off it acts like an attractant to a small amount of algae & it starts going ever so slightly green. BTW I have alot of plants crammed into my small tank with staggeringly rampant groth for a low tech, non-EI, no CO2 low(ish) light set up.
Andy