Getting a master in math after an econ PhD
It seems like a very strange route to take, and I'm not sure how it would be received since I've not seen anyone take more math after their PhD in econ.
Did you have differential equations and real analysis before starting your PhD?  If you covered those common bases, you might still gain a few useful things if you go a highly theoretical route, but you've already got most of the math you should need done.
If your interests are more empirical, it definitely does not seem like a good use of time to me.  If your interests are theoretical and you were able to keep up with PhD-level theory in econometrics, macro, and micro, it's probably still not a great use of your time, especially compared to actually getting to work in an econ field.
The best I can say about it is that it probably wouldn't actively hurt anything directly, except for the opportunity cost of publishing economic papers, and it might give you a route into doing more heavy theory.