Red tail shark eating slime coat

I've had this red tail shar for a little over a year now and he is in a African cichlid tank. Over the last few weeks has started to nibble on the sides of one on my cichlids (a star sapphire). The cichlid doesn't react much but after a bit might swim away a bit. His slime coat is starting to look a little bit whitened and he is acting a little most docile then normal. Is there any way I can stop or prevent this rainbow shark from doing it. Any help is greatly appreciated, Thank you.

 

I know with discus if a dither fish finds the slime coat you have to remove it. It will continually eat it. Stressing out the fish and not giving it time to grow the slime coat back. Which makes it vulnerable.

 

Return the fish

 

My guess is the African Cichlid isn't healthy, making it an easy mark for the Red Tail Shark.
Most African Cichlids are strong, robust fish & normally wouldn't allow a slime predator to graze on them.

 

Along with what A201 said what size tank is this and did you cycle before adding fish as that could be the cause of your fish not being healthy