I don't know why, but somehow I'm always surprised when I see you posting new footage of your hack. I guess in my mind, I always see you as having somewhat moved away from SMW hacking, so it's always a surprise to see that you've still been actively doing work behind the scenes.
I really like the level. If there's one suggestion I'd make, it's with the slugs. I think in their current form, they look a bit "stiff" and don't convey "bounciness" very well. You could change this by adding another animation to them: whenever the player jumps on them, have them get squished and then expand a lot, before returning to their normal animation. Probably takes some trial and error to get the timing right and sync this to the player's jump, but it'll be worth it. Or, even better, you could actually add two animations: The one I just described, for big jumps, and then one where the slug just vibrates a bit, for the small bounces. Would take a bit of effort, but I think it would immediately make the slugs more fun to jump on. Currently, they have a bit of "typical SMW custom sprite" syndrome. What I mean by that is that: yes, they work and do what they're supposed to do, but don't just feel quite right yet, they're not quite "production quality". The sound effect for the small bounces might also not be ideal yet, again because it doesn't quite convey "bounciness" very well. Of course the ideal solution here would be to have the same sfx as for the big bounce, just in a "lighter" variant, but I don't know if SMW has a sound effect like that, and I don't know anything about making custom sound effects, so as an alternative, I would probably just use the same sound effect for both bounces, I think that would still work better as a sound, and I think SMW does the same with its trampolines?
EDIT:
Actually, I do think the "water" sound effect also works somewhat well for the slugs, after all they're wet creatures. I still recommend not mixing the two sounds, that feels a bit inconsistent and weird. I'd stick with either just the water sound effect or just the bounce sound effect.
Feel free to visit my website/blog - it's updated rarely, but it looks pretty cool!