It looks like a very young Sea Hare, but I need a top down picture to help ID it. If it is, it's very safe and a great addition to a reef tank, they can mow through hair algae and bryopsis like nobody's business. Generally, but not always, the more subdued the colours the safer they are. The gaudily coloured nudibranchs are the most destructive and dangerous. Nudibranchs are also specialized feeders meaning they feed on one thing only and if they don't have their food source, they will die.