I just want to point out that your question implies a misunderstanding, even though you didn't explicitly mention it:

I had an HSA last year. This year I have health insurance through ACA.

The fact that you have insurance through the ACA may not be relevant. There are many plans in the ACA that are HSA compatible. But even if you don't currently have a compatible HSA plan, that just means you can't contribute any more funds to your HSA account, but you can still spend the funds already in your HSA, and regardless of your insurance plan (or even if you're uninsured) you will almost always have some eligible medical expenses that you could use the funds for.

If you don't use the funds for eligible medical expenses, you'll have to pay tax + fees on the withdrawals.

I just want to point out that your question implies a misunderstanding, even though you didn't explicitly mention it:

I had an HSA last year. This year I have health insurance through ACA.

The fact that you have insurance through the ACA may not be relevant. There are many plans in the ACA that are HSA compatible. But even if you don't currently have a compatible HSA plan, that just means you can't contribute any more funds to your HSA account, but you can still spend the funds already in your HSA, and regardless of your insurance plan (or even if you're uninsured) you will almost always have some eligible medical expenses that you could use the funds for.

To answer your specific question, no, you don't have to use the funds for eligible medical expenses, but if you don't you'll have to pay tax + fees on the withdrawals. (Unless you're over 65 in which case you'll pay tax, but no fees.)