First off, that slime is wicked. It's got to be one of the most difficult substances to draw, but you nailed it AND gave it personality. That's some pretty impressive work on it's own.
This works very well as a whole already, so the only things I've got to say are very nit-picky.
The highlights on the hair are a bit much. They seem to come from all directions and confuse my eyes. What the highlights should do, in a composition sense, is to frame or draw the eye to a specific area, usually the area strongest lit my the lightsource. The rest of the hair can have weaker highlights, which you do have on the longer locks, but they can't be so bright to distract from the focal point.
On that note, I think one eye should be slightly less detailed than the other, and one of the cheeks less bright, or have a smaller lighted area. The variation adds interest and strengthens the focal point.
The lighting on the dress is fairly nice, but the breasts bother me a bit. The highlights are hard lines making the breasts look like they come to an awkward "point". Since they are round, the highlights should also have a roundness to them, a circle or semi-circle in some cases, instead of a curved line.
You might think about adding reflective light as well. The green from the slime would bounce onto the red dress, giving it a slight green tint where they come near, and the red dress might give the green a bit of a rosy hue. Same goes for all surfaces, though some are more obvious than others. Place a very saturated object on a white surface and you should be able to see it more clearly. Adding things like that will enhance the color and naturalness of your art.
Lastly, your highlights have a tendency to look "dodged", making them look white and a little lifeless. It works for the dress since the material has a tendency to reflect light back more (white light on sequins/glitter will reflect back mostly white) but on her skin and hair, it sucks out a bit of the charm.
I notice that the slime doesn't have the dodged look to it, did you highlight that using a different technique?
When I'm not sure about what color to use for a highlight, I cheat by using a warmer color than my base that's a little brighter. So if it's blue, I add a little teal, or if it's red I add bit of orange. But different people have different approaches, and all have different merits. You should experiment some!
As I said before, this is pretty fantastic already, and I'm pretty jealous of your slime skills. I'll have to study what you did here. xD