Yeah, I would't feed hardboiled egg yolk to adult fish. Like Bill said, a very, very tiny amount for really small fish is the way to go. I use it as a starter for rainbowfish, rosy reds, barbs, and white cloud mountain minnows. Baby mbuna are pretty big, so I think they'd outgrow it after a week or so. Mbuna are (mostly) herbivores, too, so as they get older, they'd need a lot more plant matter. Haps would do better on it, I think, but, again, since they're so large, would also outgrow it in a hurry.
Personally, I think the best use for egg yolk is as a binder, per my blog post on making your own fish food:
http://www.sonnysfishroom.com/?page_id=269