Russ, I've noticed the black goo and the FA will get entrapped air in it and float. When you get a good rain it will beat the air bubbles out and it will sink. That's why it disappears. It will come up again later. I suspect the black goo gets trapped gases from decomposition and the FA gets trapped gases that it produces as part of photosynthesis. That's my theory and obeservation.

Gotta get back to fishin!