There have been several recent media reports about red tides affecting parts of Hong Kong - especially in west, as well as Shenzhen. I've seen ruddy waters from ferry nearing Green Island, and within Victoria Harbour.
Yesterday, there was pronounced red tide (caused by profusion of reddish algae) in Cheung Chau typhoon shelter. On grey afternoon, with much lap sap also floating in the water, looked very disgusting.
This was the first time I've seen anything like this at Cheung Chau. Comes after recent news reports on Guangdong using the sea as dumping ground.