Mbuna are predominantly herbivores. They might ingest a few tiny baby nerite snails while grazing on algae but they are unlikely to eat any snails intentionally. Now, I have seen some fish that don't normally eat snails, go after the meat if you want to crush the shells first. Can't say as I've ever seen mbuna do that. I'd recommend a couple bristlenose cats. They'd be faster and more thorough than most snails.
The other thing about snails is you'd better be sure you've got nerite snails and not Malaysian trumpet snails. A few weeks ago I thought I had nerite snails only to discover I had Malaysian trumpet snails. Other than cockroaches, I don't know any other species than these guys who are more difficult to control or eliminate! To paraphrase a Dos Equuis commercial, "I don't often have algae problems but when I do, I prefer bristlenose cats. Stay thisty, my friend."
