If the person performing Wudu is reciting supplications (Du'as) during Wudu, then he should not be greeted with Salam. And if it is known that he is not reciting supplications, then there is no harm in greeting him with Salam. (Fatawa Mahmudiyyah: 5/230, Fatawa Rahimiyyah: 1/124) (Kitab-un-Nawazil Vol: 15 / Pg: 241)