the way we generally determine whether this is a display issue or a Mac issue is to take a screenshot of the display, then examine the screen shot in Preview where you can move it around, shrink and enlarge.
if the spots are in the screenshot as well, then the problem is deep inside the Mac itself, possibly Memory or GPU issues. A mainboard repair by replacement is the likely solution.
if the spots do NOT appear in the screenshot, the issue is likely limited to the display module.
¿what is the date of your most recent backup, and by what method?
if you submit your Mac for Service, you will be asked to affirm that you HAVE all the backups you need, and they have your permission to ERASE everything (if needed) during the course of the service.