Scott,
Did you start the tank with the raw shrimp form grocery store? I have seen posts on a few boards that this has happened in most all cases it involves the raw shrimp to cycle the tank. If so let us know. I will tell you what others have done.
Most of the articles i have seen have said that the black goo like stuff showed up after or when using the raw shrimp. I can only figure it is from rotting.But the only fix that any came up with was to clean the tank totally. Some even had to break down and start over.maybe something was dead stuck in the LR you got and it came from there. Other than that I dont know where it comes from but it doesnt go away without work. Sorry but that is the only fix or cure i have seen.
Glenn