or minimal bg would be
+++ Art style. I like the minimalistic style. It lets us use our minds to fill in the blanks.
- The hands are a bit too 'blobby' for my tastes though.
-- On a few frames, it looks like you forgot to draw the feet.
+++++ I really like animations all about mutations and morphing.
--- But when the red thing changed, nothing all that interesting came about. You could have maybe had him using the red thing to overcome some obstacles, or maybe the red things start to put him in danger... it seems like you didn't bother thinking of anything particularly interesting, but just thought, ah, I'll show x, then I'll show x2, x3, x4 and so on...
+++ Awesome animation. Realistic movement.
Overall
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Love the animation. Maybe some texture or minimal bg would be cool.