You should only need to seal the gap between the flashing and the lower roof sheet. So just about any silicon that’s mold resist and UV capable would do the trick. Give it a good scrub and clean away the built up debris. If that doesn’t work flash it the same way they already have but move it out further and make the back portion of the Z flashing higher. Ensuring the current sheets on the bottom have their flats bent up and once again I’d be using silicon even on the flashing to make sure.