Little black scratch mark appeared in my iMac screen

If the mark is physical damage, that can't be dealt with here. If it's something like a magic marker or some other ink, gentle rubbing with a very slightly moistened microfiber cloth may help. Do not use heavy pressure and do not use anything but water. And even then, very little.

Hello,

If it isn't on the screen itself I suggest restarting your iMac, if the problem keeps happening take it to the closest Apple Store or Authorized Apple Reseller to get it repaired.

Apple Store - Find a Store - Apple

If the mark is on the inside of the screen and not on the outside surface,

see > About LCD display pixel anomalies for Apple products released in 2010 and later - Apple Support