I had a HDHP for me and dependents and my husband had a HDHP for self-only. We both contributed to an HSA. Why are his contributions being considered excess?

I can't see your private tax data, but the most common thing in your situation is that you contributed based on the Family HSA limit of $8,300 and your spouse contributed on the Self-only limit of $4,150.

HOWEVER, the Family limit covers both spouses - that is, they share the Family limit of $8,300, allocated between the two in any way that they agree to. Your limit for both spouses is $8,300, not 8,300 plus 4,150.

If you both maxed out your contributions, and your excess was about $4,150, then this is the problem.

If you have the money in your HSA(s), withdraw the excess before April 15, 2025 - this is the best way to handle the situation.