The little arrow on the files means it's an alias pointing to your source file. It should be very small in size so that shouldn't be causing you problems.
Do you have background rendering turned on. If so every time you make any changes, colour work, effects, transitions etc. FCPX will create render files that will be filling up your hard drive further, you'll end up with many that aren't even being used. If your working with long gop source files it will also be creating render files to all used clips regardless of effects etc.
If this is the case you can turn it off and delete your render files to free up hard drive space. If your specs are high enough you'll be able to edit the long gop video and manually create render files for anything that might need it. If not you can optimise your media, this will create duplicates of your source files with the ProRes 422 codec, you can then remove your original files, either delete them if your happy to or back them up to a different drive.