greenish film on inside of glass?


I have a pesty greenish film growing on the inside glass of my 90 gal. When I use the magnets it disappears in a cloud but reappears a couple days later.
What could it be? It's not hairy algae, just a thin film of very small particulates.
My parameters are great. My lighting is reduced as well as the feeding.
Excellent filtration and oversize Octo skimmer sitting in a 20 gal tank below main tank.
Help please.