"Sweet mustard" refers to a type of mustard sauce that has a slightly sweet flavor, making it different from regular mustard which is usually tangy or spicy. Full definition
For sweet mustard, soak the mustard seeds in the beer overnight. (busyinbrooklyn.com)
Don't let its color fool you into thinking that the taste is similar to German sweet mustard, which it looks like. (fieryfoodscentral.com)
Remove the potatoes from the oven when tender and allow to cool - top with more apple / cranberry relish and sweet mustard sauce to serve. (beamingbanana.com)