What is the substance turning salty water green when electrolysed?
My speculation is based on the words specifying the use of 'tap water' during the electrolysis of aqueous sodium chloride.
In particular, the interaction of formed chlorine and existing ferrous bicarbonate (found in tap water) could form small amounts of aqueous FeCl2. [EDIT TO QUOTE SOURCE]

Bacterial Iron in Well Water – Iron bacteria is usually identified by slime in places such as toilet reservoirs or by the presence of a slimy mass fouling softeners or filters.
Ferric Iron in Well Water – Also known as red water iron, ferric iron in well water is essentially clear water iron that has been exposed to oxygen – usually from the air, thereby oxidizing.  Carbon dioxide leaves the water and the oxygen combines with the iron to form ferric ions (Fe+++).  This gives the water a red rust coloring.
Ferrous Iron in Well Water – Often called clear-water iron because it is clear when poured, this substance is found in water that contains no oxygen. Typically, it comes from deeper wells and groundwater sources. Carbon dioxide acts on iron in the ground to form soluble ferrous bicarbonate. In water this forms ferrous ions (Fe++)

Per Wikipedia on Ferrous chloride, to quote:

The compound is highly soluble in water, giving pale green solutions.

Repeat the electrolysis experiment with distilled water and see if you get the same coloration issue.
[EDIT][EDIT] Interestingly, low solubility Fe(OH)2 is clear and only acquires a greenish tint in the presence of oxygen (per Wikipedia, to quote: &quot;Iron(II) hydroxide is a white solid, but even traces of oxygen impart a greenish tinge.). Further, well-water is associated with high levels of bicarbonate and low O2 levels (see this significant work). To claim that Fe(HCO3)2 is at all green is without basis, as I also could not find any source anywhere asserting a coloration, other than that it is, in effect, 'clear' as was claimed above per the comment: 'Often called clear-water iron because it is clear when poured'. Theoretical formation of the usual hydration sphere for Fe(OH)2 interacting with bicarbonate in low solubility conditions with high HCO3-, low O2, significant CO2 presence (and its associated partial pressure) suggest reasons why it may not be a valid argument. If I am wrong, please provide a single source citing color, else it should be likely assumed that my supposition is correct and Fe(HCO3)2 is clear.  By the way, Atomistry on Ferrous bicarbonate, to quote:

Ferrous hydrogen carbonate or ferrous bicarbonate, FeH2(CO3)2, exists in solutions obtained by dissolving ferrous oxide or carbonate in water charged with excess of carbon dioxide. On exposure to air carbon dioxide is evolved, and a precipitate is obtained consisting essentially of ferric hydroxide.

which one could interpret as that clear-water iron, upon exiting the pipe, liberates CO2 and forms Fe(OH)2, which, with further oxygen exposure, may display a green tint (or, per the introduction of electrolysis formed chlorine).