High Sierra occasionally shows a white/grey screen and I have to switch off and restart The picture attached shows what I see when my computer "crashes".
It happens occasionally. It may be because of heavy load. Or because
I have it open many hours. I don't really know.

Has anyone else experienced this? Can you help? How can I trouble shoot? 

Note that I have Mac Book Pro

MacBook Pro (Retina, 15-inch, Mid 2015)
2,5 GHz Intel Core i7
16 GB 1600 MHz DDR3
AMD Radeon R9 M370X 2048 MB
Intel Iris Pro 1536 MB

Note: the star shaped light area in the middle is my camera flash light. Please, ignore.
I would ssh into the box and run sysdiagnose and top to see what’s happening. 

If SSH won’t let you spawn a new process then you have a key piece of information and can press the key card to get a cyst diagnose and then look at the system logs want to to force the system down by holding the power button.