What is that weird trail? and How do I fix it? (This is my first time using live 2d)

Sprout

obsidian

Hello, there.

It's Deform Paths. I leave the explanation to the official manual.
https://docs.live2d.com/en/cubism-editor-manual/deformpath/

Ket

Looks like you have onion skin enabled.

If you want to disable it, go to Show>Onion Skin>Modeling View to get rid of the trailing images.

Naggi

It's called onion skin, and is used to see others frames of your parameter, you can disable it by going to Show>Onion Skin>Modeling View

or clicking here