Pomacentridae, Dascyllus trimaculatus (Rüppell, 1828), Domino damselfish.
Well, the pictures are terrible but they're here to remind anyone snorkeling South from Fim du Mundo beach to listen carefully. You might hear a strange kind of growl, if you look around you, you'll observe that each time this sound comes, a D. trimaculatus is attacking an other fish. As far as I remember, the agressive D. trimaculatus were not living with anemones as on the photos. Wilson reports that threespot damsellfish (from the Caribbean) "cultivates" a small algal crop and defend it vigorously by "erecting fins, striking postures, making popping or cliking sounds...". Might be that our D. trimaculatus are defending their crops !
Diving, Fim du Mundo, Nacala, Mozambique, 2006