Hawaii is dealing with invasions of several different species of seaweed (marine algae), according to this article from the Honolulu Advertiser. It's getting so bad that over the past six months, volunteers have removed more than 20 tons of Gracilaria salicornia from Waikiki reef, but they still don't have the upper hand. There are actually five major invasive seaweeds, also known as limu, threatening the coasts of Hawaii. You can read about them in detail at the Alien and Invasive Algae in Hawaii web site, created by Jennifer Smith, a grad student who spearheaded the seaweed removal efforts. Here's a link to an older article on the subject.