(c+p'ing from the email response, because it occurred to me it might be helpful for other people)
Sorry, I should've been clearer--while 'Monster' is the shorthand for the prompt, the ACTUAL prompt is "to incorporate supernatural creatures somehow"! If you want to count first contact as 'aliens' or similar, personally I would find that to fit within the bounds of the prompt--but remember that we'll be asking people to vote based on how well the entries fit the prompt, and based on feedback from last year, it sounds like a lot of people do take that pretty seriously.
So, the long and short of it is: no to the first, technical yes to the second but be aware that it might be a bit risky popularity-wise.
-mimicre