https://imgur.com/a/CrF3EcP
It seems that you can have a problem with the polarity of the signal. These green stripes must be the blanking intervals, and they have to be black.
What is this picture? Has a black background?
The scope picture is difficult to see, try to trigger well. It is the sync?
If you want to connect something to the monitor you need to what levels it uses, and impedances. For a scart they are normally 1 Vpp, and 0.3 for sync, and 75 ohms, but if the sync is TTL, of course the levels are ttl. You can even have a monitor with TTL RGB, as and old CGA monitor, but it only show 8 color with only 3 bits, if you know the monitor shows more colors, it must be analogue.
0.55 Vpp can be ok for the sync if you have measured unloaded because when you connect a 75 ohms load, the level will be half. If the signal on the scope is R, G or B, the level depends on the image, can be up to 1 Vpp or 2 unloaded, if it is scart.
And the scan rates must be the same or almost for the monitor and the source.
Scart was designed by French satanists I belive.
Scart was based on analogue TV sets, they have already the sync separator, so it was easier to use composite sync (and cheaper
)