This is not an issue and ideally you want all the unused ram to be cached. The green bar is what's used. Windows caches any frequently used files and software so that it's already in ram without having to fetch it from the hdd/ssd. Recently opened files can also be in there. When you restart it'll be cleared since ram is volatile but you'll see windows start caching it all again. Info is overwritten so it doesn't affect it negatively.