This happens often when Alan is being a complete horn dog and pushes his body past his limits when G is a little too out of it to nag him about it

Alan constantly overheats with anything that involves banging his husband so as a result a lot of the time steam just starts pouring out of his mouth as his body's last ditch effort to not cook himself alive

Also he can literally just shut down and turn off for about a hour if he overheats to much