Hi Mark,
Yes, I would say that is a form of cyno. I have been battling it on and off for the past 7 years here in Victoria. I am one of those that beleives it is in our water table, and RO/DI does help, but does not eliminate it. I would strongly recomend not using anti biotics in your tank, yes they say it is safe, but from my experience, it does have a chance of killing all livestock, and it has in my case. Chemi-clean is my choice now, it has never harmed any of my corals/fich/invertebrates, easy to use, PUT AN AIRSTONE IN WATER, remove carbon, its all good, and the cyno is dead within 48 hours. Make sure to give a substantial water change of about 30% after day 3. I know that some people have had issues with it, and I can see why, it is a heavy duty oxidizer from what I understand. I have kept aquariums for over 30 years, and cyno has been my PITA ever since the city increased our resevoir size. About the only thing I have not tried is a UV sterilizer. FWIW, it only shows its ugly head in mid/late winter, further strengthening my belief its in our water table. Whichever method you choose, good luck, and may the force be with you!
Phil