It's a red Planaria. Flatworm if you will. Uhm, good and bad. Typically it's a good detrivore, but occassionally with lots of nutrients, their numbers can be so overwhelming that they will climb over corals and smother them. A few Planaria species are known to eat coral tissue, but if you haven't seen them munching away yet, then I wouldn't worry about it. When, and if, they get into a plague proportion, you must be careful with their removal. While treating them with the same respect as if they were a nuisance algae by bumping up your tank husbandry as well as water changes and such should be able to take care of them or at least reduce their numbers to a very managable amount. Syphoning them out during waterchanges is a huge help. A few animals have been reported to much on them, but only one animal truly predates on them; the Blue Velvet Nudibranch (AKA Headshield Slug). Animals that occassionally eat them are the various dragonets, lined wrasses, coris wrasses, butterflyfish, arrow crabs, etc. If you decide to use a chemical means of irradication, be extremely careful. When Red Planaria like these die and break down, they release a toxin into the water. While the death of a few will most likely go unnoticed, the death of an entire population could crash a whole tank without enough waterchanges and activated carbon.