I wondered if it could have been injury but thought it looked more of a tumour . He's paired off with a salvini and guarding the nest so is frequently arguing with the others, I've noticed some the others are missing a scale here or there. I may have to split them up. I do at least a 50% water change every Sunday, will this be enough?It could be many things but my first guess would be excess mucus due to an injury.
Do more frequent water changes to help prevent infection and see how it goes.