It can be caused by old or poor spectrum lighting. Direct exposure to sunlight. Leaving the lights on too long. Tank temp could be too warm. Silicates and phosphates could be too high. Dissolved oxygen content in the water is too low. Using tap or other unfiltered water. Not cleaning out your filters regularly, letting detrius build up in them. Too little filtration/ too many fish. Overfeeding. Not using a good protein skimmer. Not having a proper clean-up crew like crabs and snails.
Can you describe your tank and list what your water paremeters are like ammonia, nitrite, nitrate, pH, alk, temp, specific gravity, etc?