1) To stop the Slime from listening to the players controls, switch defaults controls to "No" in the behaviour Properties, Now it will only move when you simulate those inputs with events.
2) I don't think Platform behaviour is a good choice for movement of the fish, it's falling down because platform behaviour falls down until it it on a solid surface. A better choice for the fish would probably be to create some horizontal movement with events and add sine movement behaviour, or 2 sine movements.
3. Every sprite( and every animation frame) has a collision polygon attached to it, you can see them and edit them in the sprite editor. Right now, all of your terrain sprites have a square on them, to change it, open the sprite in the editor and click on the tool/mode at the very bottom of the left toolbar( looks like a deflated, steampunk pacman ) now you can see, add, and move those points around until it lines up with the image