freezing problems plus unfamiliar screen symbol

Yesterday my iMac started freezing. It continues to do so today; now it seems, most particularly after it has gone into screen saver mode. I have rebooted in recovery mode and found no problems that needed repairing; then I did a hardware test and again found no problems. But I still have freezing problems (plus significant beach balling problems) and have also had it freeze when in screen saver mode into a black screen with a white circle with a white slash through it in the center of the screen. See attached.

Can anyone tell me what the screen symbol is trying to tell me. I can force shut down and restart when that happens.

I updated the Software this morning, but still have problems.

Thanks.