Copper ion (Cu++) is a very effective algaecide used in pools to kill and prevent algae (link to Algae page )

It's particularly effective against Black Algae, which is not controlled by normal levels of Chlorine (link)

Copper usage can, however, cause stain formation. Over time, and if over-dosed, soluble copper salts can precipitate from pool water and deposit on pool walls, turning the grout or plaster blue

Copper salts react with Chlorine to produce cupric oxide and may cause grey or black stains on pool surfaces

Copper-based algaecides and algaestats are the commonest source of copper salts for pools but Copper Sulphate is also a preventative treatment and an effective cure for many algae attacks.

It's also much cheaper than proprietary chemicals

The recommended dose for Copper Sulphate is 3 grams per cubic metre of water (5 ounces per 10,000 US Gallons),
dosed TWICE A YEAR ONLY into the skimmer, with the pump running

The manufacturers, in their helpful product labels, often suggest that this dose should be repeated monthly but twice a year is usually sufficient because the chemical remains in the pool and is only diluted by periodic backwashing and 'splash-out'

If dosed monthly there will be a build-up of Copper Sulphate; and that will stain hair, nails, grout and plaster a fetching shade of blue
Copper can be tested for with Test-strips(link) and some Test-kits(link)