ScienceAlert is reporting that a group of researchers at the University of Waikato in New Zealand have developed a "rock snot detection kit" that tests for the presence of the invasive alga didymo (Didymosphenia geminata, also known as "rock snot"). Using molecular tools, the scientists can detect didymo in concentrations of one cell/ml or less. I believe, if the protocol is the same as this presentation by the same scientists, that they have developed genetic markers for didymo and can run samples through PCR amplification to check for a match.
A tool like this would be quite useful for early detection monitoring, since didymo is most likely to spread into a new area long before it can be detected by the human eye. In fact, US researchers will probably want to get a copy of these markers to see if they match the recent didymo discoveries in the Northeast! Now if I could just find a paper published on the subject, so that I know this is not a recycled story from talks given on the subject in 2006.