There are seaslugs and then there are nudibranchs. Seaslugs are herbivores while nudibranchs are carnivores. They nearly all have very specialized feeding requirements.
There is a yellow nudibranch that eats coral soft tissue. They typically come in as hitchhikers on new coral purchases. If you like your corals, then get rid of them. Nearly every seaslug is some shade of green or brown mainly. Can't make out the color on yours, but maybe stick it in the refugium with some hair algae until it gets bigger.