Well, it depends whether I recognise it as being an alien.
If it looks human I may just walk past and keep going, possibly say "Hi!" or some such if I felt like its.
If it was big and green and holding what looked like a gun I would probably retreat (fast) to a (hopefully) safe distance and start taking pictures, or go up to it and ask whether Karneval had come early.
If it was just kinda odd and unlike anything I'd seen before I'd inquisitive and cautious. It would probably end up in me just ignoring it as part of the scenery (the great power of an SEP [Somebody Else's Problem] Field.)
I might see it and not realise it's alien, it might look like a tree, or a dog, or a dolphin, etc. In the end I expect that any alien race intelligent enough to travel to Earth will have thought through what we would do to an alien (kill it or experiment on it) so resultantly would have some very clever disguise. Well... you never know, what about the White Mice!?!