What causes clear green water in hot tubs?

Sometimes, hot tubs that are using brominating products as their sanitizer may appear to have a clear green or yellowish cast in the water. This is usually caused by a very low total alkalinity.

To eliminate the discoloration:

Test for copper to ensure the problem is not due to oxidized copper. If copper is present, treat with Stain and Scale Control according to label instructions.
If there is no copper, test the bromine and total alkalinity level.
The color will go away as the total alkalinity increases. Check again for metals due to the corrosive nature of the water.