From a study done with L. caeruleus (yellow labs):
L. caeruleus study
Increases in dietary protein level have often been associated with higher growth rates in many species. In the present study, however, the best growth and FCR for Blue streak hap were obtained using the diet containing 40% protein.
Those numbers are pretty average for most cichlids and similar to most mbuna I've seen studies on. Just about any fish has a range of protein levels they'll do just fine on, an optimum at which they'll grow fastest in their growth stages, levels outside their range that will slow growth, levels out of their range on the high side that produces excess wastes, and levels far enough out of range to cause health problems. In any case, some forum advice and hobbyist articles exaggerate the necessity of extra low protein for mbuna.
When you read science sources and some of the more accurate hobbyist articles, a lot of mbuna species are not the restrictive algae specialists some people think, but are basically omnivorous. Yes, many scrape (or comb through) algae, but diet for a number of them includes crustaceans, snails, fry of other fish, insect larva, etc. In fact, Lake Malawi actually has some stupendous midge hatches that just about anything with fins will exploit as a treat.
Some will cite intestinal length as they assert that mbuna need low protein or a specialized diet. But that's a case of hobbyists not understanding the science:
Cichlid gut length article
Our use of nitrogen isotope data to distill the complexity of tropical fish diets into a single axis of trophic position provides the first quantitative evidence that gut length varies in a continuous fashion as a function of diet. These results suggest that intestine length in cichlids reflects a trade-off between maximizing nutrient and energy absorption and minimizing the energetic demands of digestive tissues.
Not the first study to find that gut/intestinal morphology in cichlids adjusts on the fly according to available diet.
In other words, as long as you're not doing something silly like feeding them beef heart or an abundance of fatty live foods or crazy high protein, and as long as you don't overfeed, mbuna will do just as well on a reasonable diet including quality pellets as haps or peacocks or most other cichlids.