Not everyone begins Medicare coverage the month that they reach age 65, so the HSA custodian is not responsible for limiting contributions based on that assumption that you did begin Medicare coverage at that time.
Unless resolved by a return of excess contribution before the due date of your tax return, you'll be subject to a 6% excess contribution penalty for 2019 and again every year thereafter until you receive a taxable (I.e., not used for medical expenses) distribution on the lesser of the $900 excess or the remaining balance in your HSA at year end. Not only that, you don't get a deduction for the excess contribution (or, if the contribution was through your employer you must add it to your 2019 income), so your taxable distribution to resolve the excess will result in the $900 excess being taxed twice. You won't be able to apply the excess as a contribution for any year after 2019 because you will never again be eligible to contribute to an HSA. It would be a substantial financial mistake to fail to obtain a return of the excess contribution before the due date of your 2019 tax return (including extensions).
