How quickly did the lump appear?
It looks like a scale that has lifted and is covered in mucous. The fish might have injured itself and lifted the scale. If it is a scale, then maybe add a bit of salt and monitor the fish.
Salt, use 1 heaped tablespoon per 20 litres of water. If there's no improvement after 48 hours, increase the dose to a total of 2 heaped tablespoons per 20 litres. Keep salt in tank for 2 weeks.