Nudibranch/worm/??? Id help needed

[Rainer W]

Hi, i need some help idenifying this critter. It's about 3/4 inch (19 mm) long and has two short black antenna. These antenna retracted momentarily when i removed it from the tank (mixed reef, mostly softies witha few hardy hard corals like birdsnests and a few encrusting corals). Is it harmful? To the best of my knowledge, nudibranches are carnivorous.
Thanks in advance for your help.

 

[redfishbluefish]

A picture would help. Too many critters have two antenna.