Curing LR turned black after soaking in FW

[sobe]

Anyone has had this happened? I acquired some rubble LR from a fellow reefer, there are some green algea and mushrooms. As i wanted to make sure there are no pest or hitch hiker. I soaked it in diluted vinegar over night. Then rinse it in fw and let it soaked in fw changing it every 2 weeks and blasting to remove dieing cruds. That was on summer, the green algea disappeared.

Rock was almost brown some where almost white. So i let it soak more, fast forward last week i havent checked it for about 4 weeks since i was busy. I opened the bucket , the water and rocks are black. Blasted with water but rock is still black. It was not really stinky. I guess there are alot of organic matter that decayed or perhaps something else? Im Hesitating to salvage it. Was going to use some of the smooth pieces to mount zoas. Should i bleach it? Just wondering about the blacking part as i have never seen it happen before when I soaked LR