i have a 20g tall tank, i started a few months ago. i have 2 chocolate chip starfish, 1 camel shrimp, and 5 blue legged snails. and i have right now 5lbs of live rock. i came home from a few days gone and there is a tanish brown ick looking stuff growing on the rock and sides of the glass, and it is floating at the top of the water and the starfish are going to town on it. what could this be?