[Being one hell of a butler, and having the master he has, of course Sebastian is fluent in sneezese, and even if he weren't, the finger pointing would be enough of a clue as to his instructions.
Does Ciel really have to call the kitten a monster to its face, though? Sebastian puts his free hand over the kitten's innocent little ears (and its eyes and most of its head, too, since his hand is quite a lot bigger than the kitten). If this is to protect it from Ciel's cruel words or an attempt to stop more cat hair from reaching Ciel remains a mystery.]
Yes, my lord. My apologies - I will return briefly to my room to change clothes, too.
[And once he's away from the cat he'll likely realise that Ciel could do with fresh clothes as well. But first things first: Sebastian bows, before starting to head off down the hallway, carrying his kitten.]