If it is a nudibranch, it's probably bad IMO. There are lots of different species, and each species has a specific diet. That diet might be algae, starfish, anemones, or corals. Unfortunately, there's only a few that we can look at off the cuff and say "that one eats ______" It seems like all the ones that are hitchikers end up eating some kind of coral. (Look in my pictures for my 75 gallon build. I have pictures of a couple different ones I caught in that tank. Yours does not look like the ones I caught as far as species, but this would be just for comparison.)
However, before we jump to conclusions, does it crawl around? Can you get a closer picture with a higher resolution camera?