Does anyone know what this green stuff is
It has a paste like consistency.
Pretty sure that is isn’t oxidised copper as there it is too thick/ there is too much and none is on the copper pipework.
If anymore information is required let me know.
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The green stuff is cupric chloride, a byproduct of corrosion of the valve body. It's typical of water leakage where the water is of low pH. It's usually found on copper pipes. Your pipes have been painted so it's not forming on them but you do have untreated copper going into the valve. Make sure all the connection are tight.