Seems the favorite on this forum is the otocinclus - 'oto' ... they apparently clean algae as fast as it can grow.
Otos are much too weak to eat spot algae like this. They like nice soft algae's like brown diatoms. Just think, if you need a razor to get it off, how is a little oto supposed to combat it? I have otos in my tank and I have this algae. That algae isn't going anywhere until I get my algae scrubber out, that's for sure.