If you refer to the brown stains on the rocks:
It's a 4 month old tank. Normally I would say diatoms, but as you are describing tissue strands and mucus, those are probably dinoflagellates or golden algae (chrysophytes). Very feared in the hobby, but pretty normal in young tanks and the issue resolves itself in most cases over time. They can harm your CUC and corals though. Some produce potent toxins that can harm your CUC and corals can get covered in them as well. Manual removal is always a good idea.
People often associate too low nutrient levels with them. If they are the cause or a symptom isn't really settled yet.
In new tanks phosphate can drop very low as the new rocks and substrate bind it in large amounts. In that case a
DIY phosphate solution can help. If you have above 0.1ppm PO₄ I wouldn't dose anything there should be more enough. Nitrate is also often low which can be increased by dosing
ammonia or
nitrate directly. If nitrate already is at 5ppm or even higher, I would not increase it further for now.
If you have high nutrient levels instead, lowering them can help as well. Phosphate can be effectively removed with granular ferric oxide (GFO) while nitrate can be lowered by water changes and some people successfully
dose carbon (e.g. vinegar, ethanol, or specialized products in small amounts).
If you refer to something else in the image:
Probably what Mickle said a vermetid snail that uses tissue strands to catch prey.
