If you are experiencing green "algae"-like strings or clumps coming out of your counterflow wort chiller, even after cleaning, this could be down to the type of cleaner you are using.  Here is some information about different cleaners and how they interact with the inside of the counterflow wort chiller tubes:

Most cleaners work on the break down of specific materials or compounds and these can be generally broken down in to acid cleaners and alkaline cleaners.

If you are using PBW or our Grainfather Cold Water Cleaner, both of these sit more in the alkaline spectrum of cleaners.  These types of cleaners are great against some types of proteins and materials, but are not perfect cleaning solutions on their own.  

If you are using the Grainfather High-Performance Cleaner, this is based on sodium sulphate, phosphate salts and sodium metasilicate.  This cleaner sits slightly on the acidic side, which means its generally pretty good for general cleaning of things like sodium oxalate (beer stone) and protein matter, but it is also not a perfect cleaning solution by itself.  

All professional breweries conduct acid cleaning and alkaline cleaning as this is the only way to effectively ensure that full cleaning has been done.

Now, how do these cleaners interact with the counterflow wort chiller?

The counterflow wort chiller is composed of a copper coil with an outer hose of either brass or stainless steel components to connect the unit.  Copper has been used in hot-side brewing for hundreds of years due to it being:

• Very heat conductive (more so than stainless)
• Forms a durable oxide layer
• Easy to mould into shapes
• A good scavenger of sulphur in the wort
• Is an essential nutrient for yeast in the fermentation process - Copper is removed from the wort by yeast and is typically less than 1ppm in the finished beer

The Blue-green tinge you are seeing is copper sulphate, which is more soluble in high-temperature water.

When using the counterflow wort chiller in the brewing process, the copper is more conductive than stainless steel and transfers the heat from the wort to the water running in the opposite direction to the flow of the wort.  This rapidly drops the temperature of the wort to pitching temperature. 
Additionally, this copper forms a protective copper oxide layer on the surface of the copper when exposed to air.

After the boil, the wort is acidic and low in dissolved oxygen.  Due to the wort being low in oxygen, when the wort comes into contact with the copper oxide layer in the counterflow wort chiller, sulphates bind to the surface of the copper to form copper sulphate (green powder).  This copper sulphate remains on the surface on the counterflow wort chiller.

When cleaning, hot water and cleaners are used that increase the solubility of the copper sulphate in the cleaning water which is therefore removed from the surface of the copper in the counterflow wort chiller and left in the cleaning solution.  The results of which are elevated levels of copper and sulphate in the cleaning water analysis.

The copper sulphate that appears in the cleaning solution is a good thing as this means it is being removed from the surface of the copper in the counterflow wort chiller before the next brew session.  If you wish to take this further and reform just the stable copper oxide layer on the inside of the counterflow wort chiller, once cleaning is finished, recirculate a warm acid-based cleanser, like Total San, through the counterflow wort chillerr.  The acidity from the acid-based cleanser will remove all the copper sulphate and most of the copper oxide.  As soon as the counterflow wort chiller is emptied, the stable oxide layer will form, protecting the counterflow wort chiller and the wort.