The "killer algae" (Caulerpa taxifolia) and its lesser-known cousin C. racemosa are making their presence known in Croatia. According to this report from Reuters, large patches of the algae are showing up all over the Croatian side of the Adriatic Sea, threatening tourism as well as marine biodiversity. The two species apparently arrived in the area during the mid-1990s, and now cover hundreds of acres. Interested readers may also want to check out this article, translated from French by Google, about the same species.
Bonus points to Reuters for using scientific names.