Cool Idea! I like the different weapons, which is a clear avenue for more additions. I think it would be cool to have some weapons with more features, like a melee weapon that could reflect projectiles, or a gun that could remove blue paint from the ground, lots of opportunities.
It was hard at times to tell the blue slimes and arrows apart, since the slimes don't do damage I think they could look a bit more distinct, but not too much of an issue.
Having your health be labeled as ink could also have more additions, like maybe when your ink health is higher, the trail you leave behind is larger, so you would want to keep yourself at lower health.
Would also be cool if your trail did things to enemies, since then there could be more thought into where you move.
Having the other colored slimes was also cool, made my happy when I would see a red or green slime knowing I could get stronger by killing them.
Nice Game :)