Put some washes on them to mix up the colors a bit, make them look less play-doughy. Gloss varnish too, to make it glisten a bit. If you were really adventurous you could even use some thick water effects stuff mixed with a touch of paint to add a slimy coating to them.

Also, at least in D&D, green slimes are pretty small. You've got more of a green blob there!