Spokesman Henry Cambre told the Wall Street Journal the Coast Guard is testing the substance, which gives off a rainbow sheen resembling an oil spill. Test results were expected late Sunday.
On Saturday, Paul Barnard of the Coast Guard's New Orleans sector told The New Orleans Times-Picayune two calls came in about a suspected oil slick about 20 miles north of the site of last year's Deepwater Horizon disaster.
Officers confirmed the existence of a substance near the surface, but the investigation was cut short when their helicopter was diverted to a separate search and rescue mission. The Coast Guard later sent aircraft and boats from Mobile, Ala., and New Orleans to the scene.