OK CharlieD - at first I couldn't imagine how ground up mustard seeds (aka: dry mustard or mustard flour) wouldn't be Kosher. So I did a little googeling and when I became even more confused I called a friend of mine that knows a good bit about Kosher Law.
While she couldn't figure out the part about "how the mustard seeds are dried" has to do with Kosher, at first, she was able to enlighten me about other matters ... such as wine, grape juice, and cream of tartar, and how they could be treyf (not Kosher).
Dry mustards may contain anti-caking/clumping agents. Silica gel (sodium silicate) is Kosher while calcium/magnesium/potassium stearates can and have also been used as effective anti-caking agents but stearates are typically derived from non kosher fats. But, stearic acid can also be derived from vegetable sources - which are Kosher. Typically, anti-caking agents are listed in the ingredient declaration on the package. However, if a pure spice does not list anti-caking agents on the ingredient panel, based on the concept of Holchin Achar Harov (I hope I spelled that right), one does not have a halachic concern and can buy/use it and be kosher.
Something Evelyn brought up is that what might be "everyday" Kosher may not be kosher for passover - which might be what you ment about how the seeds were dried. I can't read what I wrote down ... what she called it ... but it is something about the mustard seeds being grown or dried near a wheat field where the mustard seeds might be comtaminated by wheat growing nearby. Know what she's talking about? Kamish, kemmish, kimmish ... something like that? Her suggestion, in that case, was to buy whole mustard seeds from India, wash them, re-dry them in the oven, and then grind your own dry mustard powder/flour.
Her suggestions were to read the label to see if it contains any other ingredients. She also said Colman's is kosher - although it is not certified for use during passover.
Hope this helps some.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain