They're already here and Sunny is right, not all of them are evil, just as not all of the Grays are good.
From human prespective grays are mostly evil (mostly associated with abduction cases), but they are just what they are, biological robots without soul, thus emotionless constructs. There are also what is caled a big gray aliens or longwalkers, they are organic, possibly multi-dimensional. Also, very often described as mantis aliens, in some cases reptilian. Abductees often, when they talk about their abduction incidents, talk about them as supervisors of grays. They never touch abductees, but stay aside and watch.