Yes, aptasia. I found 2 in my tank a few months ago and I figured there were probably many more in places I couldn’t see. I ordered 4 berghia nudibranchs from Reeftown, in 24 hours 1 was gone and in a few days the other. Like you I was concerned because one was in the middle of my zoas. Their only diet is aptasia. Good luck.
It is and removal should be straight forward. Using syringe, inject either lemon juice or better yet. . kalkwasser powder mixed with tank water into a paste the consistency of toothpaste and inject into the very center core and it will melt away
Guaranteed, more will show up. A beautiful solution for this is your purchasing a Copperband Butterfly fish. I’ve had zero in my tank for about 3 years. They do exist in my sump and overflow. But none in the display. Something to consider.