They seem to only attack when either I attack them or physically come into contact with them.
Is there a way to make them attack whenever they see you or is it just part of the code that they're this bad at tracking players?
Also, even after I attack them and they come after me they lose interest about 5 seconds later.
Are you using Slayer? That bug occurs if you do not have the "use all players' bricks" option enabled in a Slayer minigame.
One sec going to test things ingame (again) to make sure it's just a Slayer issue.
EDIT: Looks like Slayer is the issue. Please let me know if the issue still persists on your end once you enable that minigame option.
If you are still having problems, then the only conclusion I can come up with is that the Mutalid is conflicting with another add-on somehow.