My pool has always had these 5mm in diameter round black slightly convex deposits stuck to the plaster. I can scrape them off with my fingernail but they are not affected by brushing. They scape up as one piece...I hate to say like a scab (guess I just did). The pool is next to a rather large oak tree that does have lots of debris. Any ideas what the deposits are? They are getting much more numerous and I don't know what to do.