Another idea, if you are particularly worried, is to try feeding some pellet foods - catfish pellets or algae wafers for example - as well as flake. You may find the other kisser is simply nervous about coming to the surface - whether it's because of its tankmate or not.
Also try feeding some live foods. They should be unable to resist brine shrimp or blood worms and this'll at least let you know whether you're dealing with an aggression issue or if it's down to a lack of appetite brought on by disease.
Oh and, BTW, the kisser won't actually starve if it's eating algae. It'll be getting lots of other leftovers etc as well.