New Zealand is having big troubles with didymo algae (Didymosphenia geminata). According to this article from stuff.co.nz, the freshwater algae is spreading in rivers in the southern part of the country. The government instigated a law back in 1993 fining fishermen who knowingly spread the algae, but that doesn't seem to have stopped its progression.
The article refers to the algae's North American name as "rock snot," though from what I can tell, that is just general slang for the slippery coating on any wet rock. Bonus points go to Stuff anyway, for using the algae's scientific name.