Mbuna fry are known as an herbivore.
According to who? References?
Mbuna to a large degree eat aufwuchs. What is aufwuchs? I'll leave that as an exercise for the reader or the next generous poster, though I'll hint that it makes them omnivorous.
As for beef heart, I know that it's a popular option for some new world cichlid species, but I don't think it has much of a place for africans, mbuna especially. For africans, a protein source based on fish, insects, etc, is better than one based on mammals IMO.