Mercutio is very tempted to bite Benvolio’s ear, just for the fun of it; but the sudden arrival of Claudius thankfully (?) distracts him, and he clears his throat, disentangling himself.

“My lord!” He brightens a little, and nods towards Benvolio. “‘Tis Benvolio, newly come from Verona.” Clearly what they were doing is—greeting each other. Somehow.