Can you ground mustard seed to make ground mustard?
Maybe, If it is a dish asking for mustard seed because they are expected to be eaten whole then no, the mustard flour will overpower the flavor of the dish. If you are making pickles or something that will stew with the seed then yes, but use at most only 1/8 teaspoon dry mustard for every teaspoon mustard seed in the recipe.
As I heard from preachers about mustard seed and why Jesus pick the mustard seed for example? The mustard seed it take about 40 years just to grow the roots in the ground then after that it can grow very fast as a tree. That is why Jesus said you must have faith as little as mustard seed and what He meant was that to have deep root in faith and comes winning the war…
"Dry" mustard is ground mustard seeds, just ground up. The powder is yellow because yellow mustard seeds were used. Mustard seeds also come in red and black, and are used whole in many cuisines, especially Indian, where they are browned in oil with other whole spices before adding other ingredients. Using whole-seed spices generally provides a richer, fresher flavor.