That, I'm afraid, is Aiptasia. A glass anemone that is a huge pest, highly invasive and multiply quickly. They will sting any fish or coral it comes in contact with. DO NOT TOUCH WITH YOUR BARE HANDS or you will feel the Wrath of Kahn! Certain Wrasses or a True Peppermint Shrimp will help if they're not too large. Squirting them with boiling water helps also. There are also some treatments such as Joes Juice or lemon juice you can inject into them but I believe those are hit and miss. Whatever the case, you need to remove every last cell because even the tiniest piece left behind will grow into a new one. If you disturb them too much they release spores into the water column which will also grow into new Aiptasia. These are my biggest nightmare I'm afraid.
Well that’s certainly not good news. I’ve no idea why they suddenly started appearing. There are 2 that the center is about the size of a dime. The rest are pretty small. I’ll try a peppermint shrimp. I also read about a Berghia Nudibranch. Have you any idea whether that’s a good idea?
This one is a better pic. It’s about the size of a dime in the center oval. All but 2 are very small, maybe 1/8” in center. Only 3 about this size. They appear to whither and disappear after several days. They are all very clear/white.
I don't think Hydroids are dangerous. Just invasive. If the Nudibranch doesn't get them I think you can just pluck them out and use a paperclip, putting the tip in the hole they're growing out of and grind the crap out of them.
Well, I put 8 Nudibranch in on Wednesday and treated the largest ones with Aiptasia-X yesterday. I realized after doing a 25% water change there are far more than I thought there were. It’s a 250 gal tank and it seemed they had at least a few on every piece of rock. I am sure now that they are Aiptasia (at least most, if not all, were). They actually LOOK like hydroids but act like Aiptasia. The Nudibranch seem to disappear but, I hope they’re still in there doing something. I ordered more! I will keep this thread updated. Thanks to you both for your help.