This is a pair of Nudibranch of the species Roboastra tentaculata.
One is much smaller than the other, but they are mating. During this mating process, the bigger one was seen 'struggling/fighting' and I was able to capture in video. Nudibranch are Hermaphrodite and it seems sometimes, they 'fights' during the mating process, to determines who gets to be the male or female. It seems the 'loser' gets to be the female as being the female, it will have to carries the eggs and this is an 'added burden' as it has to get more food/nutrients to lays the eggs, whereas the 'winning male' can moves on.