Honestly, if I were in your place I wouldn’t be talking to any of them about that. As you said, you have no actual proof so even though the “clues” point towards your ex’s girlfriend, you may be wrong and accusing her of harassing you could go wrong for your side if you’re wrong.
I wouldn’t be bothering with this childishness. Unless the harassment is something worrying, ignoring that person is the best you can do. Their goal is to make you uncomfortable and to affect you with their harassment. Giving them their satisfaction and trying to solve it as if it were a problem will not make them stop, much on the contrary.
Now, if you’re dealing with serious things then you should take action, in whatever way you see fit. However, if all you’re getting are unpleasant messages or comments that you can easily identify as someone who has no better thing to do with their time, ignore it. Kids gets tired of toys so don’t give them the fun of playing with you even more.
To block the harassment I suggest you make your account private and block that fake profile. You should’ve blocked them already but if you didn’t, do it. If something’s bothering you there’s no point in letting it keep contacting you.
I also found some fake social media generators on google that people use to make fun and prank their friends like generatestatus.com. That is ok and it is for just fun.
Some times i also use fake instagram post generator to make my friends astonished by seeing the thousands of likes and awesome comments of celebrities. So use the social media like this only for fun but dont hurt others or harras others.