Doesn’t look like there’s any nipping it’s just wherever the acara goes the Firemouth chases, when the acara goes to one cave the FM follows and claims the cave and has done this in multiple caves in the tank, the acara has decided to wait up near the filter to avoid the Firemouth.These 2 fish are normally great tankmates.
As you are experiencing, all cichlids have their own personality and sometimes things just don't work.
Can you post a video or at least explain more of what is going on? Is the Firemouth nipping at the fins as well as chasing?
My experience with Firemouths is that they flare up their gills and try to look big and mean, but are more bark than bite.