The father's name is definitely William Elder, not William Joass the elder. If it had been William Joass the owner of Colleonard, it would have said "William Joass of Colleonard". Also William Joass of Colleonard graduated from King's College, Aberdeen, on 12th July 1660, so he would be referred to as "Mr William Joass". At that period Mr was not a general title for any man, but was restricted to those with a university degree of M. A.
There were two or three men called William Elder having children baptised in the parish of Banff in that period. Two have wives named, unlike the William Elder here. A William Elder with no wife named has a son, Walter, baptised in 1687. You would have to look at the original entry, but it may well be that the Walter Elder baptised in 1685 had died and the Walter Elder baptised in 1687 was a younger brother.