If the 'Cap'n' hadn't been enough, the borderline Fry Freakout was getting loud enough so that he located her direction easily. Mal's gun had been drawn from its holster, as any member of his crew screaming had a way of rising that kind of reaction from him. Though as Mal's quickened pace led him into view of his mechanic and her companion -- Mal couldn't see just what was making her shout.
Of course Mal suspected the worst of the other man right off the bat, but that instant malice was fading upon taking note that the dark-haired stranger wasn't so much as touching Kaylee at the moment. Neither of them were running off and there was no sign of any kind of struggle. Just Kaylee.. with her arms over her head.
The gun was shoved back into its holster, Mal was at something of a loss.