What can this be?... how do I fix it?

How big is your tank, what is your vacume/water changes schedule ? How much do you change ? Any water test results ? That should give us a hint if its water quality .

A hint if its nip damage is the front would likely be acting a bit stressed. Hiding, having the pale blanched out look on the blue stripes, hanging out at the top of the tank. Lots of darting around. Overall very nervous looking compared to the other fronts. At least thats what mine do.

Fronts are lazy fish compared to haps and peacocks. They will often claim an area and just "hang out" there, only moving to chase away an intruder or avoid the alpha. Mine do not swim about the tank going back and forth, they tend to stay in one spot except for the alpha , he patrols and inflicts pain and suffering LOL.

BTW, if he is very nervous and jumpy i find leaving the lights on for longer times everyday helps along with steady traffic or visits by their owner will help to make the fish less shy. Avoid netting if you can as well. Give lots of krill treats too. Krill will make you their friend LOL. Get some freeze dried krill and pop it in some warm water then feed it to them when soft. I noticed that when i leave the light off for more than 2 or 3 days and suddenly turn them back on they tend to revert back to nervousness. i especialy noticed this with my mbuna but fronts too.