What category is "cash" under when I make a journal entry for sales tax collected?

I don't think cash is involved, given that you have the cash for the sales tax and sales, and if all was recorded using a sales income account, then that likely the offsetting account.

It would probably be preferable to edit the deposit and split the gross sales amount between the appropriate income account(s) and a sales tax liability account, and then you don't need a journal at all.

@minersgrabngo 

In this case, "cash" is your bank account (asset) that Square deposits to.  The term "cash" is used interchangeably for actual cash received and deposits to a bank account.