After a late startup this year (due to some retaining wall construction) we finally got in this past week, but noticed the walls & floor of the pool felt very rough. Just brushing up against it or kneeling on it was painful, and has produced some nasty scrapes on the kiddos. I've attached photos (taken w/ my trusty $99 Nikon underwater camera). What the heck is it? Is it safe to swim in? How do I get rid of it? Any pointers welcomed, we're a little freaked out.