Can I use my HSA to pay a medical bill that is in collections?

You can use an HSA to pay for any procedure that was performed after the HSA was opened.  If the procedure that is now in collections occurred before you had an HSA, it’s not an eligible expense.  

Can I give my actual HSA card number to the collections agency? Or do I have to pay it on my credit card and then reimburse myself?

You can give them the HSA card #.