The white growth has been spreading rapidly across coral reefs in bays on Kauai's north shore, affecting up to 40 percent of the coral in some bays.
Other areas are "just as bad, if not worse," biologist Terry Lilley, who has been observing the phenomenon, told the Los Angeles Times.
Scientists at the U.S. Geological Survey say the growth has been identified as both a cyanobacterial pathogen, a bacteria that grows through photosynthesis, and a fungus.
Deadly to coral it strikes, the invader is moving at the rate of 1 to 3 inches a week on every area of infected coral, scientists said.
"There is nowhere we know of in the entire world where an entire reef system for 60 miles has been compromised in one fell swoop," Lilley said. "This bacteria has been killing some of these 50- to 100-year-old corals in less than eight weeks.
"Something is causing the entire reef system here in Kauai to lose its immune system."
Experts said there is little they can do to stop the threat.
"It's very alarming," Wendy Wiltse with the Environmental Protection Agency in Honolulu said. "All of us are concerned about it. We want to do more. Part of the problem is we don't know what to do, especially in the case of a disease that's spread by a pathogen. It's not like we can put antibiotics in the ocean."