How old is your tank, that would have alot to do with it and whats the current chemistry of the water. It sounds like diatoms, if it's a new system once it's finished cycling the diatoms will cease, a good way to help get rid of it is to take a sponge and wipe the glass but remove it from the water then rinse off, if you only brush it way with a scrubber it will get into the water column and only spread further.