Keep that guy but keep it mounted on its own rock. If left unchecked it will literally overgrow your entire tank!
i would touch it to see if you get stung than you will know for sure hahahah
I have no position to advise as I never have chance to keep them. Decision is yours, think due to the nice color morph there I keep them with caution. They are from Hydrozoans family come in red, blue, purple, orange most common brown. Do read this article for your consideration.
http://ibmi.mf.uni-lj.si/acta-apa/acta-apa-01-1/4-clanek.html
If I may ask, where can I get a red, blue, purple or orange morph?
My LSF did bring in the colorful morph looked like fire coral last 2 weeks but they call them ausie lace coral. Goggled thru I discovered both of them are from same family of Hydrocoral but I believed of different species. Thought of the same capability of stinging I try avoid buying one. Wish I have snapped a photo of the colorful coral.