I haven't been able to establish a connection between Johann Valentin(e) Clem and Adam Clem, but I think there's circumstantial evidence that supports that there may be one.
Johann Valentine Clem died in 1791. Three people were mentioned in his will: Catherine (his wife) and Henry and David their sons(Reconstructed 1790 Census of Georgia (De Lamar & Rothstein). Johann's will mentions Henry, David and all my children which opens the possibility that Johann had more children.
Catherine Clem went on to marry again.Her second husband was William McGehee of Baldwin County Georgia. Catherine died around 1846.In her will she mentions a daughter Elizabeth Rose (wife of Washington Rose). Elizabeth was born about 1790, so she can't be the daughter of Catherine and William McGehee as some people assume. I believe that she is Johann and Catherine's daughter. A Mary Clem is also mentioned. She is identified as a sister. That doesn't make sense as Clem is Catherine's married name. Prehaps she is an unmarried daughter of Johann Valentine Clem from an earlier marriage.
Johann Valentine was born in 1729. I have no concrete proof but I believe based on dates he was probably married twice. Per Dee Clem, Johann and his wife Caterna are listed in Deed Book U 1769-1771, p 17, of St. Paul's Parish. Johann would have been 40 at that time and Caterna would probably have been around the same age. Catherine Clem McGehee lived until 1846 so there's no way that Caterna and Catherine are the same woman. That would support the theory that Johann had an older set of children by Caterna. Adam would have been of the appropriate age to have been part of that family.