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Ok, do you have a white cup? Plastic maybe? If you can dip a little water in it, that should get you a definite answer on the color. If it is brown, some water changes should straighten it out.
How long has your tank been set up? Is it cycled? Do you have fish in it? Could be a bacterial bloom if it isn't cycled. Could be an algae bloom esp if the tank is near a light source. Or if you are overstocked, it could be "dirt" from the fish.
What are your water parameters? Have you tested your water?