The water has a general haze. The colour matters: white or grey is different from green and from brown.
Short answer
White or grey haze in a new tank is usually a bacterial bloom and clears itself. In an established tank it usually signals a change in the biological balance and is worth investigating rather than filtering out.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: Established tank; follows a large substrate disturbance, a dead animal, overfeeding or a filter clean. Often accompanied by an ammonia reading.
How to tell: The haze is distinctly green rather than white.
How to tell: White or grey haze in a tank set up within the last few weeks. Water smells normal, fish are unaffected.
How to tell: Appeared immediately after setting up or rescaping; the haze is dust rather than a milky cast and settles overnight.
How to tell: The tint is brown-yellow and the water is clear rather than hazy — like weak tea.