I am having this problem at the moment, it appeared from nowhere, then suddenly there is a purple ring around the top of the water and purple dirt at the bottom of the pool. The pool shop people here in Spain say it is a fungus and it has to be treated slowly. First they put granule chlorine in the water, then the pool had to be brushed to break down the membrane around the fungus, then the skimmer had to run for as long as possible. This was done yesterday, the large dirt has increased, but apparently that is a good sign, and there is very little new pale purple marks. I had to run the skimmer again for a long while and then leave it. Tomorrow if there is no new occurence in the pool it will be vacuumed, then retreated. All going well by the weekend it will be clear. But I have been told this is one fungus which can't be got rid of overnight.