Yup agreed. Looks like aptasia. If you can I would remove the rock, pull the aptasia off with tweezers, rinse the rock with salt water (maybe with water change water) and put the rock back in the tank.
If you cant remove the rock there a hundreds of ideas people have. I’ve tried using kalkwiser powder mixed with a little tank water to make a thick paste. Grab a 10 mL syringe suck it up and slowly squirt a little paste in the mouth of the aptasia. Make sure the aptasia grabs the paste and eats it. This normally kills it.
There are other fish and shrimp that eat the aptasia as well. Peppermint shrimp and worked for me. I’ve heard some file fish eat it as well.
Lastly don’t just pluck it out of the tank or disturb it. It could possibly send out seeds and you will see more in your tank.