That look like a form of dinoflagelatte...
I know this is not the treatement for dinos, but try a drip in iodine, then into peroxyde. Peroxyde will kill the dinos on the zoa.
Before my first dino outbreak, my zoa turned brown like that. Take a turkey baster to try to remove the brown slime. It worked with a toothbrush for me. Watch your eyes when doing this, this will stress them and they may squirt toxic liquid.
I'm saying dinos because of the tiny bubbles on the brown slime. On the first pic I see a stringy slime, like a dino... If you have what I think, it's a real good pest, I'm fighting them since 2 months without results.