If it doesn't look like a sea hare, it may be a nudibranch. Most nudibranchs are harmful in reef tanks, and if you do see it again, I would catch it, and put it in a tupperware container with some holes punched in it until you can ID it. That way it will stay safe and alive, in case you find out it's something good, but if it's something bad, you can just take it out.