They declared a state of emergency over this, are spending a ton of money and effort to try to get control of it, and emphasize how it is disastrous to native ecosystem.
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Officials have not confirmed the source of the infestation, but said in the news release that it was most likely caused by someone emptying a saltwater aquarium into the bay, since the seaweed is a "popular and common saltwater aquarium plant."
Emptying into the bay would include doing a water change drained to the street (rather than into the sewer). This is how our hobby gets banned in California.