SSD still full of media after moving projects to other disc
Working on a very large project and needed to make room on my SSD to finish it. I moved 5-6 other projects off of the SSD and onto another drive, but the SSD still shows 75% full. When I look at the contents of the 1TB drive, it shows 178GB in the Final Cut Library, but there are two other folders on the disc named "Final Cut Optimized Media" and "Final Cut Original Media" that are filling the drive up to about 75% full. I would really like to make more room on the disc to speed up the workflow but don't want to jeopardize my previous work.
What are these files, and why didn't they move along with the projects and events I moved to the other drive? How can I safely get the ones that don't pertain to my current project to where they need to go?
iMac with Retina 5K display, macOS 10.13