I don't think that coralline acts as much of a barrier towards the rocks filtration ability. I have read both that it does and doesn't, but I tend to think that it doesn't, at least not to any large extent. It is an algae, and as such it isn't that much of a barrier against water and nutrients from flowing throught it. It also only grows where light hits it, so there is still plenty of rock surface that wouldn't have coralline growth no matter how much you may have. Also, since it sort of 'smooths out' the rock surface a bit, food bits and fish waste tend not to stick to it, so it helps in the filtering of detrius that might otherwise settle into the rocks pores and crannies. In my own opinion, rocks covered with pink and purple, while certainly pretty, just don't look natural. However, I will take a good growth of coralline over hair algae any day.