It's possibly doing a shimmy or a 'display', basically advertising it's dominance, an activity that many mbuna's will do, especially top-tier males.
Some mbuna's will follow up a shimmy with a charge (or attack) directed at rivals or may commence breeding with a receptive female. The display (which appears seizure-like) demonstrates the cichlid's vibrance and vitality. The movements broadcast waves which other mbuna's in the vicinity can detect, and I think I recall they also release (and fan) pheromones. They will also fan out their fins as they do it.