The small globs of oil were spotted around noon Wednesday.
Los Angeles County lifeguard officials closed the stretch of Manhattan Beach from 34th Street to the pier as a precaution. The Hermosa and Redondo Beach coastlines were also closed.
In total, the mysterious oil globs have impacted a 6.8-mile stretch of coastlines.
Lifeguards are working with the California Department of Fish and Wildlife, U.S. Coast Guard, L.A. County Department of Beaches and Harbors, L.A. County Fire Department Health Hazardous Materials Division and County of L.A. Public Health Department to determine if the oil-like substance is a result of an oil spill or a natural occurrence.
"They don't even know the cause, so everything is precautionary at this point," Manhattan Beach Mayor Wayne Powell said.
The agencies collected samples of the product and are investigating the cause, composition and the source of the substance, officials said. The U.S. Coast Guard is also working with Ocean Blue Environmental to clean up the oil globs.
Officials were asking residents to avoid contact with the water, wet sand and any materials that have washed ashore. The oil may cause skin irritation and long-term health effects, officials warned.
This unusual oil slick comes about a week after a ruptured 24-inch pipeline spilled oil into the ocean off Refugio State Beach, about 20 miles west of Santa Barbara. The pipeline, owned by Santa Maria-based Plains All-American Pipeline, was on land, and the oil flowed from a culvert into the waters.
Anyone with further questions or concerns was urged to contact L.A. County's beach advisory hotline at (800) 525-5662 or visit www.publichealth.lacounty.gov/beach.