Weird blue and green dots on my iMac screen. I can’t not move off it

IF you are really running 10.11 as your profile says my guess is the iMac is pretty old, please tell us what the model year is and confirm it is running 10.11.x. If it is, then my guess is your iMac is telling you it's time to be replaced as the display or the GPU may be failing. If you have an external display you can connect, please do and then test with that. If the artifacts appear on the external display then you know for sure it is the GPU and if they disappear then you know it's your internal display.

Other things you can do is to run Apple Diagnostics or Apple Hardware Test (run both at least 3x back to back) and see if any errors come up. You can also restart the computer in Safe Mode and then restart normally and see if there is any difference.

Take a screen shot & view that in Preview, move the Preview window around, if you see double lines it's a Video card/VRAM problem, if the lines do not move moving the pic around then it's the LCD or cable.

Lots of 1 to 3 year old iMacs having screen issues.:(