These files are all compiled versions of other files.
TL;DR: The file names are generated not choosen. The files in both of these screenshots are essential and not suspicious. There's no reason as of writing to modify anything in the storage directory.
The .php files are what's called "views" (or blades in their u uncompiled form) - they are the html and PHP templates for the different physical pages you visit on the forum. You can view all the ones Flarum itself provides here: https://github.com/flarum/core/tree/master/views. The ones with the weird names are views that have had the relevant data injected and are stored to save processing time, clearing the cache forces these to be regenerated. The names are just hashes as far as I know and are not meant to be modified in their compiled stated.
The JS and CSS files are the combination of Flarum's core JS and LESS and all of your extensions JS and LESS. The LESS (which is a more feature rich version of CSS) is converted into true CSS which your browser can read. These files are deleted and regenerated when you clear you cache or enable/disable an extension.