Not bad, but needs more retooling
Interesting enemy capture system, similar to (if not inspired by) Kirby.
It's good how you made a "trail" of yellow and green balls to indicate where you're supposed to be heading. Basic platforming design, though-and it could use more road signs on certain places.
The character is also cool, as there aren't too many games where you play as the slime. On the other hand, there's a bunch of RPGs out there where you slay them by the dozen!
Finally, the enemy capture system that is at the heart of the game functions properly.
Also, I didn't know at times where to progress, as some parts are dead ends and require you to beat up a certain number of enemies before proceeding. For example, how was I supposed to know that the point to the right of the first switch that you find is actually one of these dead ends that allow you to progress? I killed the first batch of enemies there, and the screen won't budge. Only by killing a few more will you be able to progress.
Oh, and in said area, it takes too long for enemies to spawn in some of the earlier dead end areas, so for a while there, I actually backtracked, thinking I was trapped or that I overlooked something. There were several other instances in the game where there's no clear indicator that you're supposed to beat up a set number of foes to continue, yet it's one of those conditions that have to be met.
The simplest solution to avoid confusion on these parts is either to block out the way backward (probably not the ideal solution anymore in these platforming games) or to simply add an arrow marker pointing in that direction. In fact, wherever it seems to branch, just place a marker.
The variety of regular is lacking, which leads to a repetitive feeling. Then again, I understand why, for each normal enemy you make, you would need to make another form for Globby, complete with a new attack. (It's double the work, really).
Level 2 has a glitch (maybe) in which you can actually jump over a tall vine blocking the right side of the map (either with the double jump power or by luck from the mushroom nearby) and fight the boss, thereby bypassing all the find the five crystals stuff. You can go left past the gatekeeper and then not be allowed in like normal without the prerequisite crystals (I actually did this on my run through the stage), but the boss will still be active.
By the way, you aren't totally invincible when you use the Flame or Tornado attacks (Just try hitting another tornado slime in its attack animation with those attacks and see what happens to your health.) But they sure make stages and boss battles a lot easier, as they allow you to inflict more damage (only three hits from Flame rush would kill most enemies) and make you move substantially faster. You could do that or get the triple shot attack or other projectile and just do hit and jump away tactics to hit the boss.
Arcade (survival mode) was too easy and seemed just tacked on for the fun of it, for after consuming a Tornado or Flame Rush slime, the score is only limited to the player's patience and avoiding attacking tornado slimes without a high amount of HP.
A minor side note I had was that the music got a bit repetitive.