Since July 2021 I suddenly started encountering a problem on my otherwise trusty S9+, where the screen is a dark and/or greenish with glitchy green bars/lines on top when I wake the phone I've seen other reports of this problem -- not just with the S9+ -- and I'm convinced this is not a hardware problem but a bug due to a software update. Fortunately, I've found a somewhat awkward workaround that is 100% effective when I use it.
Samsung, please work on a fix for this for your customers!
There appear to be two "modes" in which the phone turns on its display -- I'll call them Mode A (the phone's normal mode), and Mode B; Mode A is where the display glitch lies, but Mode B still works well. (I don't know the technical names or purpose for these two modes, but I'm guessing a recent phone update affected one and not the other, maybe using newer function calls or a display API that the S9+ doesn't recognize.) Mode B occurs when you activate the phone from the "Always On Display." The "Always On Display" (which is a somewhat confusing name) is the state where the screen shows a simple clock when the phone is at rest.
The default in my case was that "Always On Display" was triggered by tapping the screen when it's asleep. So if I tapped my phone to get the simple clock screen first and then pushed the power button, the screen would go into Mode B and work perfectly; conversely, if I just pushed the power button from sleep, the screen would go into Mode A and be dark and/or green-tinged.
Fortunately, you can change the setting so that "Always On Display" is actually always on, so that Mode B is always activated*. To do that, go to Settings > Lock screen > Always On Display and make sure not only that it is set to "On" but that "Show always" is selected.
Now, as long as I turn on my phone from that simple clock screen*, the display will work perfectly. If I don't, (such as if my phone has been asleep and I get a call) I get the dark/greenish screen.
*There is a caveat: Sometimes that simple clock screen appears a little weirdly tinted, which indicates to me it's in Mode A. If I wait a few seconds I can actually see the clock colors change a little, and then I can press the power button to turn on the screen in Mode B and my phone works well.
An additional characteristic of this glitch is it appears to be related to screen brightness. Sometimes my phone screen turns on in Mode B but is usable ... although everything is green-tinted and washed-out; but if I increase the brightness it will quickly become dark, glitchy, and unusable.
So now when I my display is dark/green/glitchy, here's what I do:
1) click the power button to turn the screen off
2) wait for the simple clock screen to appear (and possibly wait a little more for the color change)
3) click the power button to turn the screen on
4) swipe up to get to my home screen and use my phone as before
I hope this helps others who are similarly frustrated with their Samsung phone!