i am always worried when mbuna do not eat, it means something is wrong..
the times i have had mbuna not eating have been 3 examples...
1. Fish holding (obviously with eggs, they want to eat, they just can't

)
2. New fish that are un-used to the food type provided, give it time they'll be ok when hungry.
3. First signs of bloat. which then if untreated can wipe your tank out..
I hope it is just one of the first two..
Bullied/Streesed fish can become weak and get bloat, this is cuased by a bacteria in the gut of the fish, the fish refuses to eats, or eats then spits in out, also takes on an 'out of breathe' look.
This fish still excretes which spreads the bug and all mbuna nibble each others poo (bad i know but true) then the bad bacteria spreads to the other fish...
What colour is the poo of these none eating fish then?