Nearly pulled out the blow torch!
Looks like remains of a barnacle with some stringy stuff sticking out. Grainy pics, below:
A bit better visualization by eye. Doesn't look like polyps from close up, but I've now got that spot on watch!
Good eye!
The eye? I developed it working on Aiptasia/ mojano cleansing project in this tank. I had a hitchhiker rock that infected my tank. I had glass anemones that I injected with lemon juice, H2O2, and rinsed with pool acid that survived like they had been to the spa for a skin procedure. It’s been a crazy fight but I think I might be winning?