Hi Marilyn,
Since you mention Altenbruch and Neuenwalde, I am guessing that your Mulsum is the village/parish in Land Wursten near the North Sea -- this is commonly called Mulsum I in some records. The records that I looked up and posted were for Mulsum II, the one between Bremervörde and Stade.
I believe I have a line of collateral Buck relatives that settled in the early 1700s in Neuenwalde (or maybe it was Debstedt just south of Neuenwalde). I also have family from Otterndorf several miles east of Altenbruch. However, I do not know of any connection to the names that you mentioned.
I do have ancestors on my father's side listed in the Mulsum II Ortssippenbuch from the mid-1600s and another family on my mother's side who lived there for a short time in the 1770s.
Fred Buck