I don't recognize it by itself, it might be helpful to post a full tank shot and someone might be able to identify what it belongs to in your tank. Is it fluid filled? Do you have macroalgae in the tank? Any fish missing a kidney?
It's Botryocladia. I threw a bunch of random macro algae scraps in this clown fish tank over a year ago and there was just a bare stem at one point from the botroyocladia. and a lot of it started wasting away, then I dosed nutrient with iodine a few months ago. Red macro algae loves iodine and low light apparently. I have a carpet of this stuff now.