I don't know what is wrong by looking. I know this is no help but that is the very reason I use products that won't stain. I'm using Dyna Gro and it's clear in the water.
Perhaps empty your water, scrub the tank, line and bubbler with bleach then refill. You can add a product H202 that helps with slime/rot. I've been using 5-7 drops of bleach per gallon lately without problems.
I'm not going to tell you to add bleach to your water but if that was my stuff I would scrub it out, add fresh nutrient water and a few drops of bleach per gallon.
I lost a nice plant to rot and it was because of high temps. Prevention is so important, I really watch my water temps now.