An NBCSanDiego user, Phillip Schneider, said he owns a plane and has been noticing the foam up and down the coast for around two weeks. He snapped a picture of the foam floating on the surface of the water near Oceanside.
No sightings in the Bay Area - yet.
Surfers have been discussing the strange foam on messages boards online.
"Yellow lines from Swamis to Seaside at a level I don't recall ever seeing before," john4surf posted online.
Another surfer with the screenname "justintime" wrote, "25+ years of surfing in OC/SD and have never seen that color before."
Last week, researchers at Scripps Institution of Oceanography found Tetraselmis, a microscopic green algae in samples at Scripps Pier.
The fluorescent foam is still around this week, but is only visible at some beaches. The foam becomes more apparent in the afternoon because of the wind and waves.
This green flagellate which is only 10 micrometers in size has been found in concentrations as dense as 15 million cells per liter of seawater, according to Scripps researchers.
There's no documented health hazards with swimming or fishing near the bright yellow/green foam.