What is this black mossy moldy like stuff building up on some of my ornaments in the fishtank? I think it may be the same stuff that builds up on sunken ships in a large lake or sea. Is it a kind of bacteria that crawl on surfaces and produces waste that the filt doesn't take? I have a 29 gallon aquarium with 6 fish in it. I like to know what the heck this stuff is before I do a water change and wash the ornaments. Thank you