That is an asterina starfish. It's a love / hate thing, where some love them and some hate them. I have a tank full of white / light tan ones. I've been told that the dark ones are the ones that will eat corals, but the whites just cruise around my glass cleaning off the algae.
Asternia starfish. You will find mixed reviews as whether they bother corals. I haven't had any issues with mine bothering corals. They reproduce quickly and can become a nuisance if the tank has excess nutrients.
Thanks so much for the responses , I believe I saw him once before about a month ago, (much, much smaller). My ... It's grown! Lol
I'll keep an eye on him..
Mine are the white/tan ones as well and I've found them time and time again on my zoas eating away. So when I do see them doing this I put them in my sump.