Can I use 2018 HSA money to pay for qualified medical expenses incurred in 2017 when the person is no longer going to be on our medical plan as of Jan 2018?

Money in an HSA account can be used for eligible medical expenses even if the expenses were incurred in a prior year.

But only if the expenses were incurred after the date on which you established your HSA.  The date that you established your HSA is governed by state trust law, but is generally the date on which funds were first contributed to your HSA for a year for which you were eligible to contribute to an HSA.

Also, these 2017 medical expenses must be expenses incurred by you, your spouse or one of the dependents that you claimed on your 2017 tax return.

Yes, we already had the HSA established before the expenses and it is for an eligible person. Thank you.

Thank you bsch4477 and dmertz for the information.