Ì find I get algea with certain fertilisers & not letting the soil dry out enough (Which is mainly a problem with backed clay products as substrate)
Uhmmmm... That's another probable cause. You'll see the first time it showed up it was right after I had thoroughly fertilized my ficus a month ago (as ficus keep growing on winter I fertilize it once a month with a balanced fertilizer). And at first instance something ressembling a white mist or a thin web appeared on top of the soil. That was, as I said to Dave, when I removed it using a knife, but just a week ago it started to "resprout" again... That is why I was thinking that it was not directly related just to an excess of water.
Alain