Just my small nitpicks.
1. A foot looks like it's turned backwards in one frame of the run, not sure where.
2. Also, it looks like he's trying to hold his hands together. Maybe make them more apart?
3. I actually thought that one of the legs didn't move. Then I was like, 'oh, it's a gun!'. Maybe have him hold it somehow?
Just some small minor nitpicks. Otherwise, really good. Rock tiles look good by the way. As a quick way to turn those into grass, just overlap your grass tile with it. You could also break up the rocks into smaller bits if that's what your going for on grass.
I like the slower slime animation. The fast one is too jerky in my opinion. I definitly like the hair piece. Really nice!