|Yes, those are their legs. Lots of hairy, sticky legs.|
Some barnacles fan the water, back and forth, back and forth. Some unroll their long legs, wave them once, and immediately roll them back inside the shell, over and over. Of these, each one has his own rhythm, some faster, some slower.
These tiny ones mostly just keep their fans spread out, retracting them briefly from time to time to see what they've caught. Copepods, baby crabs, and leftover shrimp crumbles, probably.