IMO the vertically-striped mbuna (kenyi?) seems to be in the early stages of challenging dominance by vigorously attempting to maintain a perpendicular stance vs the apparently dominant horizontally-striped melanochromis (auratus?).
The perpendicular stance is a way that upcoming challengers keep both their mouths and caudal fins away from another fish's mouth (i.e., their 'primary weapon'). To me the kenyi isn't quite yet read to engage in a lip-locking contest (which I think is what the auratus is vigorously trying to initiate). A loss at a liplocking bout can sometimes permanently assign a fish in to a sub-dominant role.
The kenyi doesn't want to present its caudal fin (tail fin) either because that gives the melanochromis assured dominant position (pursuer position). A mbuna which is chased has a hard time becoming a viable challenger or potential alpha fish.
Alpha fish can find this stance (which presents its opponent's broadside) to be provoking and frustrating.
In a sense the kenyi is 'stepping up' to the alpha fish while attempting not to present any vulnerabilities. This is essentially a first-stage 'contest of wills' (the kenyi has to maintain the perpendicular stance while the m. auratus has to attempt to undermine it and some how engage in lip locking or a long range pursuit (to cement its position as alpha fish).
This behavior resembles courting activity (which involves circling and intermittent pauses), but it seems apparent to me that the circling in the video is the auratus getting an opportunity to get at the kenyi's caudal fin and the kenyi adjusting its position.
In time (if the kenyi is able to maintain what it is doing) the kenyi will gain enough confidence to challenge the auratus (which will culminate in skirmishes and/or lip locking.
I'm wagering both participants are maturing juvenile males (if I'm right the kenyi will become a near solid yellowish fish and the melanochromis will become a black horizontally striped fish).
If I'm wrong then one of these fish will be mouthbrooding before long!