Did you find out what this was and/or manage to get rid of it? I have exactly the same thing happening in my axolotl tank (only 6 weeks in!) I know exactly what you mean with the 'dandelion like' and moving with the water. Worried it might be hydra though as it does change when you touch it, but don't know. Any extra comments/thoughts would be really appreciated - have attached a couple of my pictures (equally hard to take a photo of though :/) I checked the water quality today and is fine.