That looks like the algae slime I contend with. Does it sort of peel off like a big slimy mat? When I started my first fish tank and had no idea what I was doing, I saw it forming and thought it looked cool, and within a month it literally coated 50% of my tank like a tiny aqua-golf course or something. Once I realized it was not a good thing, I found it virtually impossible to get rid of, hence my terror this time around. I think, in my case at least, since my parameters are all good that maybe the tank needs more aeration? Do you have an air pump going?