Looks like it's sloughing. If the darkened areas are coming off in thin sheets/strips that'll be the case. Most new soft corals will often go dormant and shed when placed in a new tank or even do this to shed impurities from the outer layer even in an established system.
As long as you aren't seeing any sign of actual necrosis, you should be fine. Be sure it gets good water flow to help with the shedding process. You can also help it along later when it's nearly done by using a turkey baster or the like and ever so gentley blowing off the bits that might remain. Don't take that step unless the coral is almost done though or it could irritate it.