100% aiptasia anemone. They can be really difficult to get rid of and can spread like wildfire and kill other corals. Read up on eradicating them. The best luck I’ve had besides removing and/or scrubbing them off the rock, is to smother them with superglue. The are pretty bullet-proof, so even those methods aren’t guaranteed. There are some fish and inverts that supposedly eat them, but generally only really small ones, and it’s not a for sure thing plus they sometimes get an appetite for corals. There are a lot of schools of thought on the best way to get rid of them, but most people usually have at least a couple in their tanks, and one or two isn’t going to do much damage to your blastos, but its their potential to spread that is worrisome, so even if its not visibly upsetting your blastos, I would still try to get rid of it.