I'm thinking this is just a normal consequence of not having a tile band (see pics), but thought I'd better check it's not something more sinister. This look mainly happens as the tide goes out and as you can see it's mainly limited to directly below a mortar join in the coping. If I brush the walls (or even just splash some water) up the sides it mostly disappears which is why I think it's just the plaster drying out a little more than the rest of the plaster above the water line... almost like the mortar joins are sucking the water out of the plaster. I'm just a little concerned that it might be the other way around i.e. something is bleeding out of the mortar and may leave scale? If the pool is really full it doesn't really show up due to (I assume) the capillery action of the water soaking into the plaster up to the level of the coping.