I think theyre yellow polyps, but its a tough call.
I've seen Jeff's get in 20-30 rocks of yellow polyps that were all a translucent dark yellow color (almost brown) before. It depends where theyre collected, I suppose.
Most aiptasia I've seen have longer, stringier tentacles, although I know there are alot of types of pest anemones we call "aiptasia". I also have never seen aiptasia crowd like that, as others said above.
Is the "mouth" raised or does it fold in? I agree touching them is a good way to test. Polyps close differently.
IME, yellow polyps can be pretty irritating to other corals in their own right, so I would watch it either way.