Decals change the colors of the sides of your player when they stretch too far out of the boundary of space that is actually used by the body. As you may have noticed, when you make a decal, some of it gets cut off. This is because the player's body doesn't actually use all of the space.
You can easily fix the discoloration by cutting off some of the excess stuff. Most of the excess from decals is from the bototm, which (by looking at your decal) you have a lot of. Cut off some of the bottom of the decal and just leave it empty and the problem should go away.
Also it's very clear that you didn't draw this as it's super fuzzy and low quality. You should start drawing your own decals.
