Looks to be a bacteria of sorts.
What to do is make certain that your water parameters are pristine. The tough bit is, there will often be nutrients that are utilized by the organism that won't show with testing simply du to the fact that they are being utilized by the organisms. Test your source water and your mixed water and tank water. Report back. Additional info is helpful in diagnosing issues so if you could give us as much information on your system as is humanly possible, it would help.
I have no idea. I’d recommend a UV filter. What livestock do you currently have? Seems not much. Can you take the rocks out and clean and perhaps take clearer pics? A microscope would help ID. Never seen anything like that to be honest.