I find the behavior of that guy suspect! 
Years ago i was on a "non-official" Naturist beach in Fuerteventura and i remarked a strange man strolling around on that beach and the entire coast. He was in his mid40ies, always dressing shorts +T-shirt right in the middle of naked people. I saw him nearly every day walking by, sitting down and watching the people. Maybe he was unemployed or bored....but i never have seen him bother people. Possibly he was just a simple Voyeur!
The good thing was to see even Spanish people naked at the beach, even if in the minority.