Screen cleaning issues

There is no easy fix for a screen that has been damaged by cleaners and/or improper cleaning.

You're best bet, is to take the iMac into your local Apple Store or AASP and get an estimate.

For the proper screen cleaning method, see > https://support.apple.com/en-us/HT204172

What cleaner spray was used? Did it damage the glass or just leave a nasty residue on the screen? If the latter you can try what I use: eyeglasses cleaner sprayed on a soft cloth till lightly damp.

Perhaps there is a local AASP that is closer to you that can look at it.

see > https://getsupport.apple.com/?caller=grl&locale=en_PR