What is this brown clumpy stuff? See attached pic

[garycordell]

We did not look at our spa for several weeks. When I opened the cover I found a lot of brown fluffy clumps attached to the sides (see pic). I have never seen something like this. It brushed off easily, so not a problem. Just curious what it is. We live on the Big Island of Hawaii.
Thanks, Gary

 

[cowboycasey]

If an empty spa I would say it is starting to mold, who knows what kind but it seems to be growing nicely

I would throw some bleach on it and rinse well...