It is short but cute. The graphics are relly nice for a first game!
The difficulty is rather casual
When not using full screen the edges are cut off for me.

I've used unity before and for a first attempt at a 2d platformer using it this is really good.

Congratulations on finishing your first game!
Even though there is space for improvement, what you have now is a pretty solid game in all regards, well done.

The main thing I'd suggest to improve is art and level design. A big thing in pixel art is making sure “a pixel” has the same size in all artwork, for consistency. Here; spikes, trampolines and moving platforms have the largest pixel sizes (making them seem low-res), then the main character and portals, then the detailed trees and static ground (making them seem high-res in contrast).

As for level design, gameplay-wise it is pretty good, not so much aesthetically.
The interactive/scripted elements seem to be scattered around the level somewhat randomly – i.e. they are exactly where you need them to traverse the level, but they don't seem very “cemented” in the world, like they are really meant to be there if this was a real world.
Especially in Level 3, it seems like everything floats in the sky without rhyme or reason, even more so when it's coupled together with the pixel art consistency mentioned above.

I feel the main character's jumping animation is slightly off.
If I imagine how a slime should behave when moving:
1) Going wider on the ground in preparation for the jump (which now happens mid-air)
2) Jump, moving up taller than wide (which now happens only for the trampoline)
3) Normal shape when reaching the top of the jump curve
4) Falling, being taller than wide again
5) Hitting the ground and being wider
Something similar is shown here: https://youtu.be/haa7n3UGyDc
You have all the frames you need drawn already, they just need to be timed and used better.

Additional suggestions for improvements:
– Background music would help a lot

– If you fall into a bottomless pit, should you play the death animation, or simply fall below the edge of the screen (the former implies there's solid ground or a row of spikes just below the edge)

– Is there any point when you *don't* want to be teleported when touching a teleport? They are one-way only, too (even though the blue one still plays the sound). So why have a button prompt at all and not teleport automatically?

– In some levels, you start behind the edge of the screen – and sometimes you're left, sometimes right – so it may take you a sec to figure out where you even are

– You set Newgrounds to a very large resolution, but your game actually only uses a fraction of that. And a part of the text (like “Level 3”) is cut in the standard resolution, anyway

In Unity, you should anchor the text to a corner of the screen, and then if it's set to be “10px away from the top right corner”, it will look the same in all resolutions.

– Level 5 (both standard & fullscreen resolution) has both teleports off-screen, and you have to guess where they are

– Quit button is pointless in a WebGL game and simply freezes the game (if you wish to quit, you close the tab) – instead, you should have a Main Menu, Restart or Level Select button here.