Cyano or Corraline??

Peysonnelia is a coralline genus. If that is it, (too early to tell what species it is from the picture), it will be calcified. If you do the calcium test described before you can either rule it out, or if it is calcified I would say it is a type of coralline, whether or not it is in that genus though will take more time or a microscope at this point.

 

Even though it is only moderately calcified it would produce a + vinegar test? What do you look for? Just pour vinegar on a spot and see if it dissolves?

 

Coralline should show bubbles in a cup of vinegar. Scrape some off, place in a cup with vinegar, monitor for the first few minutes, give it some time, monitor again, repeat. If you see bubbles forming it has calcium, is not cyano and is likely coralline. No bubbles, it is not coralline, and therefore likely to be cyano.

- edit - you can setup a control with something you know is calcified so you can feel assured it is working correctly, and another cup with something that is not calcified so you know bubbles won't form from just anything.

 

I have some of this stuff on my rock too. I always thought it was old coraline of some sort as it was on the rock when I got it and the rock had been dry for months. I never bothered to try to scrape it off, but it looks just like the pictures posted in the beginning of the thread. Hope its not harmful...