Design critiques?

That's very good artwork.
Technically, i would eventually remove/darken the kind of fresnel reflection on the bottom right of the ball, because it causes noticeable aliasing.
And the kind of teeth are very small and look a bit like noise or dithering. Eventually remove them or make them larger if possible.

TheLuckyDonut said:
Something just feels a little strange when compared to the metal shell.

The ball looks like a skull, but when it opens, the inner being has only one eye, while the skull has two. So we wonder, why would use such one eyed being a metal shell made for two eyed creatures?
But there is a difference in expression too. The closed ball looks sad because of eye slant. The slime being inside then looks curious and motivated. So again the inner being does not really fit its shell.

Though, that's no problem of there is more of such senseless / mutated stuff to see in the game. If it matches the theme, it's fine.

Maybe the closed ball looks too sad, so we do not associate it with an enemy or treat. I want to help it, rather than shooting it. Maybe you want it this way. It's not necessarily a mistake. Is often done that way in games and it can work, e.g. if the player is meant to figure out which creatures are dangerous and which not. Even if there is no related mechanic about good and bad NPCs, it can be good to give the player a mood of doubt and uncertainty.