Why are your water parameters, what lights and schedule do you have, what kind of flow, what size tank, what CUC do you have. All questions we need answers to to help solve the problem
Could be dinoflagellates. It looks like stringy brown snot with bubbles. There is a sticky thread on this. You don't need to read the whole thing, just the first post for making a positive ID and treatment recommendations depending on the exact type you have. From what I have read it usually occurs in low or ultra low nutrient systems.