the alien stands out too much from the background.
No, he doesn't?
I think he stands out just right in brightness.
However, I think the colors are much too saturated. If you desaturated the background a bit, you could make the character stand out so much more. Right now, it feels a bit as if he's fighting for attention with the background, colorwise. Subtlety can be very valuable.
Everything in the background is terrestrial, so there's nothing about it right now that says galactic to me, except for the alien. Is he on Earth or another planet? If it's the latter, I suggest changing the colors to make it look alien. You've already established that they're trees and bushes, which leaves some room for creativity, even if he's on Earth.
Your color ramps also seem random in how they change in hue and brightness. If the connection between two colors is not clear, it can make the image seem disjointed. Your brain has to try to figure out if they're parts of the same ramp or an entirely separate ramp.
Otherwise, I really like the design. Especially the asymmetrical head.
