Try oxalic acid, Mary Kay On-Off or the Starbrite cloneyeah, i can tell from micro-molecular structure that it's.....
you know, it's kind of hard to tell from a picture...
i have Interlux VC Performance epoxy that will get brownish from being in the water... get some Bon Ami or Bar Keepers Friend and give an area a quick scrub to see if it cleans up
If On & Off doesn't work, nothing will.Try oxalic acid, Mary Kay On-Off or the Starbrite clone