Whatever it is, I had tons of the stuff. It seems to be something planted tanks, especially those with aqua soil, go through as part of the settling down process.
I guess you'll feel the slime on the tank glass? If so then the word from the forum about my similar situation was that it was biofilm.
If that's correct then it'll be a good source of food for the algae eaters you have, and it'll stop renewing itself when the tank settles down.
A word of caution though. Following the biofilm stage I then got a damned awful problem with hair algae. Watch out for any signs of that and nip it in the bud as it's really difficult to get rid of.
I have stopped using my overhead lighting right now and the only light that hits the tank is from a nearby one and from the early morning and late evening period when I open the curtains. It seems to be working.