Worm identification help

[CaledonFish]

Does anyone know what the heck this thing is? Found him crawling on my star polyps. I have removed it from my tank, but I'm worried that there may be more.
Are they harmful?

Any help is appreciated!

Sorry for the crappy pic. Only had my iPhone handy.

 

[Brilliant_Barbs_21]

Can't really tell because it's so blurry but it looks like it may be a nudibranch of some sort.