Iron bacteria are a natural part of the environment in most parts of the world. These microorganisms combine dissolved iron or manganese with oxygen and use it to form rust-colored deposits. In the process, the bacteria produce a brown slime. You can poke the top oil looking slick and if it break apart in sharp edges, its the bacteria not oil.
This was spotted on the Bittern Marsh Trail at Lake Lewisville Environmental Learning Area, Lewisville, TX.
Thiobacillus ferrooxidans and Leptospirillum ferrooxidans are the bacteria that feed on iron. This was shown to me by a retired geologist. I took the photo several years ago, but the bacteria is still in the wetlands along the trail.