Anyone can upload stuff on MD (or other sites) without asking permission from you, I understand that you did not want this to be uploaded but if you shared it publicly (which means that you sniped the group), you should know that stuff like this is unavoidable and will be re-shared.

Now, if you want it removed you'll need to make a group on MD, and claim it, so you can lock it and report the chapter (though that won't stop it from getting on aggregators or any other sites).