Scratch

That's because the character sinks into the ground, then is pushed back up, then sinks back down, etc. I believe that it's actually unnoticeable if you enter the stage view, but you can also fix it by making the sprite not go up when touching the ground. The disadvantage, however, is that the sprite will go right through upward slopes.

For that you'll need a sensor sprite that looks something like this:

The sensor follows the player around. If one side is touching something like a wall it sets a variable (such as right_hittest) to 1. Then on the player sprite, make it so that it can't move right if right_hittest is equal to 1.

I'm not sure about hiding it. The standard way is to set the ghost effect to 100%.

thanks