مُسْتَرِيحٌ describes a person who finds rest. مُسْتَرَاحٌ مِنْهُ describes someone from whom others are relieved after his departure. To clarify these two expressions, the companions ask مَا الْمُسْتَرِيحُ وَالْمُسْتَرَاحُ مِنْهُ؟.
The pronoun هُ in مِنْهُ refers to the person from whose harm relief is experienced. In the first expression the subject himself receives rest; in the second, others receive relief. This is the key contrast in the wording.