I gave a mystery snail to a friend who has a very algaerific (my own word, thank you) tank. She was always a big eater (the snail, not the friend) and every time I ask how it's going, the report is that she's constantly on the move gobbling up algae. At first there were clear trails where she'd plowed through it on the glass. Now she's got it to a point where it's pretty cleaned up in there. I didn't realize a mystery snail could do all that, but I see that it depends on the individual. There's another mystery in there who doesn't eat half of what she does.
I have nerites that do a sparkling clean job. About once a month I give them a vegetable, and once a month some shrimp pellets. Any more often and they start slacking off with the clean-up program.