I'm sure it's anti-aliasing which generates without asking blend between your red and green in such a way that the antialising zone is darker than colors around it. You can make the blending happen so that the result is less offensive. An example:
In the left there's 2 rectangles with your red and green. The image is rasterized to low resolution to make the dark antialiasing zone well visible.
In the middle there's the same rectangles as vector. The green one has got a narrow stroke which is the same green, but desaturated. Desaturating is easy in the colors panel HSB mode.
In the right the vector version is rasterized. The antialiasing zone is not darker than the rest of the shapes.
If you are not going to rasterize anything, know that rendering your image just to show it on a display does it without asking. But a grey stroke, thin enough and as bright as the green area fixes the dirtiness also in that case. Draw it with the Pen as a separate path to limit it on the red area.
You can find older cases where automatic blending between colors creates an unwanted dark zone. The fundamental math reason for it is the the non-linear gamma of the ordinary sRGB color space. The problem is non-existent if one uses either some linear gamma version of RGB color space or selects in his image settings option "make blendings in a linear gamma RGB color space". Unfortunately my ancient Illustrator offer none of these. But here's one discussion of it for Krita: How to get rid of weird black borders in Krita?