Water discoloration can be caused from metals and algae. When metals come out of solution but have not "stained" the pool surface it can cause the pool water to take on another color such as: pretty green, purple, brown, etc.
Many swimming pools eventually get a buildup of "waterline rings" which are caused from non-living organic contamination. Over time such things such as; sweat, lotions, oils, vegetation, soaps, etc. build up to cause waterline rings.
There are numerous conditions that can cause cloudy pool water, such as: algae, dead algae, small particles not getting trapped in the filter, poor circulation, heavy bather loads, excessive non-living organic contaminants, unbalanced water chemistry, and high calcium levels.
Calcium Hardness is an important part of water chemistry. If Calcium Hardness is too low it can cause the water to become corrosive and can result in the etching of pool surfaces. High Calcium Hardness will result in scale formation on pool surfaces as well as in pipes, filter, etc. It can also cause cloudy water and eye and skin irritation.