The Haplochromis genus are omnivores, they eat both vegetable and animal foods. The larger species in this genus such as Aristochromis, Rhamphochromis, Tyrannochromis and even some Nimbochromis are Predatory which means they need fleshy foods such as shrimps, mussels and prawns but all of the Haplochromis genus will eat any flake, pellet or stick food given to them. For this reason it is best not to keep the big Haplochromis with the small Mbuna genus.
The Mbuna genus are Herbivore, they eat vegetable based foods such as Tetra Pro Veg and Tetra Pro Veg Crisp. There are many good quality flake, pellet and stick foods available and Mbuna being the greedy cichlids that they are will readily eat anything you give them. There are also specialist foods produced for Malawi cichlids however always read the packets as some are just standard fish food.
If you do feed them non vegetable based foods then it is always best to include some form of vegetable or Algae in with their diet. This can be done by offering them pieces of cucumber or shelled peas. The cucumber will float so attach a piece of led weight to it or use a "Screwcumber" to weight it down. When feeding peas you can buy any cheap frozen peas from your local supermarket. Leave a few to defrost, once defrosted split and remove the shells then drop them into the tank.
You can always try feeding them with Spirulina flake or a Spirulina based food however I have not had a Malawi cichlid that has liked Spirulina yet. Remember vegetables will go moldy so always remove any uneaten left overs from your tank and use vegetables as a supplement to their normal diet. Mbuna are vegetarians and can not properly digest fleshy foods so avoid giving them foods mentioned above for the Haplochromis genus diet.
Another food to try is the homemade recipe on the Malawi food recipes page. You can even try adding fruit and see what your cichlids prefer. One food that you should avoid feeding to your Malawi cichlids is frozen foods.
There has always been a bit of a debate as to whether these foods cause Malawi bloat. This is the swelling of the fishes insides making them unable to swim. This always results in the death of the fish as there is no cure. If you are going to feed frozen foods then make sure that they are properly defrosted before feeding and always feed it in moderation.