You are not getting part of your ex-wife's military retirement. She gets the full amount of her retirement from DFAS. DFAS is not paying you. She is paying you. The amount she has to pay is a percentage of her retirement, but the payment comes from her. It doesn't make any difference whether she sends you a check or a Zelle payment. If the divorce decree does not say that it is not alimony, then it is alimony. (I'm not sure why that's helpful, but that's the rule.) Double check with your lawyer, with the caveat that many divorce lawyers know a lot less about tax law than they think they do. I don't see any need to contact DFAS. You have no connection with them.
Since you were divorced after 1984 and before 2019, you report it as alimony income on your tax return.
For all the details about alimony and taxes, see IRS Publication 504.
