Ya, it’s a type of calcified algae. I tore a 16 year old tank down because it was a plague. I found out I could have just tossed in an urchin. Usually goes away but sometimes becomes a plague. My PSD from seeing it would have me toss the snail but you could probably scrape it off outside the tank and then scrub the spot it was on with a toothbrush and a little hydrogen peroxide.the second pic is from my old tank.
Remember, mine was a rare case and even if you see more popping up you can toss an urchin in. I didn’t have all the resources back then that we have now. Happy reefing.