That’s normal wear. AppleCare Plus covers accidental damage if the watch doesn’t work any more. What you’ve got is extremely minor cosmetic damage, which is not covered.
That minor scratch on the Apple Watch aluminum casing is hardly noticeable, and Apple is not going to replace your watch because of that. Even if they did replace your watch, you will likely scratch it again because it’s a watch and it’s prone to being damaged being on your wrist. I would just use it and enjoy it for what it is. Also, if the aluminum is scratched or damaged, it’s permanent.
In normal use, watches do get scratched. But also in normal use, neither you nor anyone else is likely to notice a scratch like that. Live with it. You’ll probably soon have more to keep it company.
Most won’t care either if it does scratch. Its a tech watch, not a nice Swiss watch that has some unsightly scratch on it. It’s meant to be worn and used, and almost impossible not to have some type of scratch being worn daily on the wrist.