When Robin looks up at Phalanx's fingers, so does Phalanx--getting distracted by their own digits. As they continue to follow along, they become absorbed in rubbing at the discoloration that seems to be a possibly permanent thing for them, picking at the edges of the nails where it may always look as if dirt is lodged underneath.

And in response to Robin's question? Phalanx just laughs, as if it's a pretty a good joke. What a stupid question...