Carbon is adviced in all reef tanks, specially mixed reef, to take up the chemicals corals give off to prevent a chemical war between them. soft corals do give off ALOT of chemicals to fight for space and light and ... .
Carbon will also remove alot of heavy metals and impurities which could be the cause of this !
I'm gonna ask if you have done any bigger than usuall water changes or if you have lowered nutrition in tank alot suddenly ? when I was experimenting with some Zeo additives, I reduced nutritions alot and boom the millis were Pxxxed off lol I solved it by adding Zeovit Xtra which is a mix of many different corals foods, other coral foods may have the same effect.