I have read that an HSA account can be used to pay for dependent's medical expenses but it did not specify if the dependent had to be on the HDHP as well. I am with a health care sharing ministry.
Yes, distributions from your husband's HSA account used to pay your unreimbursed medical expenses are qualified medical expenses that make the distribution tax free. As indicted in IRS Pub 969 (https://www.irs.gov/pub/irs-pdf/p969.pdf), if you are the HSA owner (your husband is the HSA owner in this case), qualified medical expenses are those incurred by:
1. You (your huband) and your spouse (you).
2. All dependents you claim on your tax return.
3. Any person you could have claimed as a dependent on your return except that:
a. The person filed a joint return,
b. The person had gross income of $4,000 or more, or
c. You, or your spouse if filing jointly, could be claimed as a dependent on someone else's 2015 return.
(Note that you are not your husband's dependent. You are his spouse.)