how do i remove green flashes on final cut pro x

Green frames are sometimes an "artifact" of certain .mp4 encodings. Find your clips in the Event browser, right click on them and Transcode Media to Optimized. That will usually take care of it.

We need more information.

Do you see these green flashes in specific clips?

What type of media?

What version of FCP X and macOS?

Do the same green flashes appear when you play back in Quicktime Player?