I have had him in my tank for 5 days the first time I noticed this was last night. I have never kept one of these before so I have no clue if it is growth or dye off. It is at about 180 par.
Growth. They will show a darker purple rim where they are growing. If it's ****e that's when you need to worry. Also make sure there is nothing close to this guy. They have sweeper tentacles that are unbelievably long and pack a powerful punch. I'm talking 6-8 inches long on larger ones.