Hi, sorry as I don't member the name of it, but I saw it some time ago and they suggest keep it, as isn't anything harmful.
I'll keep it now until some one from #reefsqaud or someone else don't come with the proper ID for it. I do Love mollusks like this (nudibranchs, flatworms etc) keep it separate from the tank until you figure it out.
BTW I'll close the container (watertight) to keep the salinity in exact the same as it's been, not to kill it, just in case it appear to be good creature