Could be brookynella... In which case, I pitty you... I have never seen Brookynella myself. Given the fact that it is not purple and the color is from your lights, brookynella would be the best guess. If so, I know little to cure the fish. It has been said that following this routine works best:
1) Giving a "dip" in freshwater with methylene blue of the same temperature & pH as the Marine tank, for about 15 minutes ( careful observation must be made, during this time, to avoid distress, & the fish removed, if major problem is observed).
2) Giving a bath in Sea water with Formalin added add at a dosage of 1000 ppm for some 15 minutes.
3) Adding an Acriflavine product (such as Fish-Vet's Revive) to the tank water for a period of 2 weeks after the above treatments.
4) Taking severe steps to ensure that water quality is optimum along with the TOTAL removal of any detrital matter .
5) Using a Quinicrin based product.
I suggest carrying out all this from a hospital tank. You need to remove all the fish a let the tank run empty for about 6 weeks.
I would submit the photo to make for sure, but I think Jmaney could be right.