We have had Diamond Brite on pools at 2 different houses (3 yrs in one and 4 in another) and have had some orange colored spots that I think is due to iron deposits. You may want to have your water tested. Once in balance (if that is the problem), the stains will probably go away if you rub them with a wire brush. If anyone else has a more solid diagnosis, please let me know!!
All in all, I have found Diamond Brite a thousand times better than marcite. It's easy to care for, tough and uses less chlorine. Good Luck.