90 gallons with 25 total fish.
25 mbuna's in a 90gal; I think that is a good quantity of fish and not a lightly stocked mbuna tank. Adding more is an option but shouldn't be necessary unless you're adding more female yellow labs.
The yellow lab is likely a male attaining maturity exhibiting conspecific territoriality as well as communicating its dominance over all others of its kind.
Chasing is all part of normal mbuna behavior. It's when the fish being pursued is singled out, pursued all over the tank and/or is physically brutalized (nipped caudal fin, body scrapes, etc) that there is a serious problem. From the sound of it he's chasing any yellow lab, not a particular one.
In time the other male, as it mature's, may begin effectively defending itelf, or the dominant male, confident in its established status, may shift to breeding-mode with the females.
EDIT:
..My other yellow labs are all in hiding, behind a heater, one is behind the filter intake, one hiding in a plant and the other is hiding behind my magnet cleaner....
What's your rock-scaping like? The area's that their taking refuge in kind of indicates inadequate rock cover.