I must confess to being a bit puzzled about the issue here. To me, it looks like this:
Aliens: I assume what is meant here is intelligent life elsewhere in the universe. Since we have proof of life here, whether we think it was 'created' by a deity or not, and it seems there is evidence of planets elsewhere with conditions which may support life, or at least stars which could produce such planets, it appears to me that the probability of life elsewhere can be estimated.
So - we know it's possible, because we exist, therefore there must be a probability it also exists somewhere else, and given the size of the universe the probability seems 'high' to me. It doesn't seem unreasonable to me, on this basis to be of the opinion that aliens probably exist. Call it a belief if you like. Whether they have visited earth is a different question entirely.
God: it seems to me that much of the evidence to support the existence of a god is interpretable in ways that don't imply a god too. So belief in god seems to me to be much more debatable.
I don't see that belief in god and belief aliens are comparable, or that atheism and acceptance of life elsewhere in the universe are incompatible.