I'm not sure what could be done. I mean, other than a color swap, the similarities are a bit too much.
I'll straight up admit right now I was inspired by slasher somewhat with an oc of mine, but the "result" is far removed and further removed when I had it worked on more, and I never even referenced what you did in the long run/used any images.
https://www.furaffinity.net/view/14812708/ and https://www.furaffinity.net/view/17228570/
However, what I am seeing there is someone too lazy to even try and are like "I gotta more moody edgy version" and I mean some artists have similar designs but it's just...too close. Lazy. Not sure if it's worth worrying about, but leave the link here anyway. Maybe someone will tell them to put in some effort-if they can draw, they do better than a poor imitation of the real thing.

So I should ignore it?

Also I like you're bat art I always like see others putting effort and hard work into creating and that my work inspired that.

Well, no. I was just really saying it's up to you in the long run. Is what there doing right? Definitely not. Just not sure if it's worth a giant effort over someone using a former look poorly imitating is all. By all means, go after them if it's really bothering you. It is definitely AT LEAST borderline plagiarism at this point. So the "not sure if it's worth worrying about" part, like nobody whose really a fan whose been watching, sees, etc.-will think they are you. However, because some won't, it is worth at least addressing them, I believe. Where you go from there is a decision for how other fans feel and you to make and only you and others. For what it's worth, I think it's wrong.
And thank you, I appreciate that despite being inspired, I clearly didn't well....do what this lazy poor imitator is basically.

Thanks I think I'm going to keep an eye on this, if nothing changes or goes to far then I'll address it.

Tbh he doesn't look THAT much like slasher, maybe give him some different physical trait, like some marking, or a scar, or a detail that would make him stand out more ^^

I’m not familiar with your characters, BUT, a creator retains the ownership rights until they hand it to another person & with explicit proof, eg a journal + sales invoice etc. A character doesn’t become ‘up for grabs’ when the artist uses it less.

You could send them a DM and just say hey, I’ve noticed you’ve heavily based your character on one of mine; the similarities are too close & it’s made me uncomfortable. Please consider altering your copy with distinct differences, thank you. Colours that are slightly lighter or darker are not enough.

Thanks I actually talk to the person today and it's been resolved.

I guess one factor you have to consider is the age of that person. There is a phase of heavily relying on knock-off / fan art / fan characters from which eventually more original looking characters and styles can develop over time as learners try to figure out which stylistic choices and shorthand fit them better.
Clobbering them with copyright is a bit too early, unless there's a merch operation underway.

How were you made aware of them? Are they following you on Insta? Did others point them out to you?
If anything, an ambiguous "I see what you did there" kind of action should be done, so they'll at least be able to figure out, that you are aware of their existence. Maybe posting a comment of unsolicited constructive artistic advice to one of the pictures they eyeballed from you or just the silent follow.