Hey! Very good question ! I just want to tell you at the start that if you are not an expert in Greek language or Biology then you don't need to know that . Everyone use these two words as synonyms. But ρύπανση is used to speak about the pollution in the enviroment that comes from human intervention but μόλυνση is about the pathogenic micro-organisms that can be exist and develope because of the pollution or something else . For example I can say ηχητική ρύπανση ( sound pollution ) , ρύπανση των θαλασσών από πλαστικά (sea pollution form plastics ) but you can not use ρύπανση to say about an infection of a rusty nail on your foot for example ,then you must say : μόλυνση από σκουριασμένο καρφί . i hope i hepled!