I am in process of replacing a shower - I have exactly same model to fit.

The water pipe and the electric cable (by design) come through a square hole in the back of the shower and a corresponding hole in tiles and plasterboard. I have had to dig out most carefully all the mastic to get at the water coupling. I find that the earth conductor has gone green for a short distance where the white sheath of the cable was cut back and the earth conductor was not covered by green/yellow sheath. Because this effect was buried in the mastic I suspect it's due to chemicals in the mastic. I also suspect, but of course cannot be sure, that the mastic is plumbers mate. The earth conductor has cleaned up ok but I'd like to prevent recurrence. What mastic should I use? Would taping the area with electricians tape stop seepage of solvents in the mastic? or could it be that solvents might penetrate thin plastic tape)

There isn't any slack cable to pull through so that the mastic would only be in contact with outer white sheath of cable.

Copper goes green if there is any sulphur about eg in the form of air polution from cars or burning coal. I don't know if there is any in mastic.