Is It Ok To Erase Everything In /var/custom_Files/sess_Data/?

/var/custom_files/sess_data/ got too big. It's the biggest folder of the entire website.

It's more than 60% of the disk space used at the time of writing this.

Is it ok to erase all it's contents as part of maintenance?

Yes. They are temporary image files that are supposed to get deleted automatically at a certain point.