I assume you're talking about processed and packaged dead sea mud? If so, I'd just put it in the checked bags, as it would probably fall into the same category as hand lotions and whatnot.

If you're talking about just going down to the Dead Sea and scooping up a handful of mud into a baggie, you may be running afoul of the US Customs prohibition on importing soil (http://www.cbp.gov/xp/cgov/travel/va...cted.xml#Soil). You could in theory argue that dead sea mud doesn't meet the CBP definition of soil, since nothing really grows in it. But I'm not sure your average CBP goon would agree. And there is probably SOMETHING alive in there.