The boy appeared out of nowhere - one minute not there the next there. Naturally Pietro stated moving towards him, because he wanted to know who it was that just showed up out of nowhere like that. Oh he had suspicions about who it was -
He came to a stop because Wanda told him to, and then told him to leave the other man alone. It was only a temporary stop he told himself, because he was trying to do what Wanda asked as she was in the condition that she was in.
"Who is this?" Pietro asked that question of Wanda pointing towards the other person.