I have about 95% Haps in my tanks and they go absolutely crackers for prawns! I do have a few Afra in their and they dont really bother. They will occasionally take a bit. The Frontosa's love them too.

But I haven't really heard about prawns not being good for Mbuna TBH. I know bloodworm is bad and can lead to Malawi Bloat but prawn is a new one on me but then again I'm not a fan of Mbuna so probs wouldn't of heard it.

You learn something new everyday!