Mine is most fond of diatoms (brown). We had an outbreak in my goldfish tank, and it was gone in a couple of days (and he's still pretty small.. only a couple of inches!). I've had to feed him algae wafers since, he kept a 29 gallon algae free for several weeks, and since transferring him to a 55, he's been keeping that clean too.
He's not very picky, as far as I can tell.