Dear reef friends, I am a beginner in marine aquariology, I am writing from Italy. DSB tank of 1 year and 10 months with soft corals, anemones and some LPS which is perhaps beginning to proceed with dignity in recent months. After finding and removing some small orange flatworms on the base of a bastomussa I occasionally check the tank with a flashlight with the lights off. Last night I found on a rock, near xenia, the flatworm (or nudibranch?) visible in the attached photos. Length about 3cm. Do you know what it can be? Is it harmful to the reef? Can I leave it? Thanks in advance for the advice!