The charge will be on there until the pre-auth expires. I find with my MBNA card this takes a full month but it can be much shorter. The hire company can fix it, but it’s unlikely they will.
The best thing to do is to tap on the transaction in your feed, scroll to the bottom and tap ‘something wrong’. Pass all the info you have on to Monzo and they might be able to return the money quicker.
Regarding the bit above, hopefully that was confirmed in writing rather than over the phone? If not write to the company and get them to say the same but in writing so that you can send the evidence to Monzo. I’d also try and get them to confirm that the rental is complete and no further charges are to be applied.
Also, as others have said I’d strongly recommend using a credit card when booking car hire in the future.
Make sure you come back and let us know how you got on.