The discovery Wednesday night of the substance later identified as linoleic acid forced closure of a stretch of the beach in Santa Monica.
The discovery of the substance was reported about 7 p.m. between lifeguard stations 15 and 17, Santa Monica Fire Department Deputy Fire Chief Tom Clemo said.
The substance was identified by hazardous material experts from Los Angeles County. Linoleic acid is found in nuts and fatty seeds and has cooking and industrial uses.
The material was possibly coming from a septic tank, perhaps being discharged from a cruise ship, according to a Los Angeles County Fire Department dispatcher.