You lick it.

Its tip is shaped like a cock and looks like an anus, and you know how to please both of those. Tentatively you let your tongue slide out of your mouth, circling it around the alien cock in front of you. The cockhead shudders, presumably with pleasure as it clenches. Then you turn your attention to the asshole sphincter, lapping your tongue across it before pressing and pushing at it, trying to work your way into it so you can please it with thrusting motions. It feels so strange and somewhat humiliating, pleasing a cock and eating out an asshole at the same time. Especially that of a strange alien creature that's half forcing you to tongue fuck it.