In this scenario, Aang has indeed missed his trigger. Eldrazi Mimic’s is fairly unique among Power/Toughness changing triggers, because the words ‘you may’ give it a choice on resolution, when other such triggers generally don’t (prowess, for example). When we check the IPG concerning triggers with choices on resolution, we find that:

A triggered ability that causes a change in the visible game state (including life totals) or requires a choice upon resolution: The controller must take the appropriate physical action or make it clear what the action to be taken or choice made is before taking any game actions (such as casting a sorcery spell or explicitly taking an action in the next step or phase) that can be taken only after the triggered ability should have resolved.

In this case, by passing priority to move from the main phase to combat without mentioning the trigger or a P/T change, Aang has missed the trigger. As the trigger is not generally detrimental, there is no penalty. Briefly educate if necessary about why this trigger is unique, ask Naya if she would like the trigger placed on the stack, then have the players continue with their match.

Thanks again for joining us this week, check back later this week for another splashing scenario!