Ej22 coil corrosion

[SuBaRiNo]

I noticed last week that one of the plugs in the coil were the lead goes in was badly corroded. I spent ages cleaning it up but it still is no were as clean and shiny as the other coil outlets.

Although it's as clean as humanly possible im worried this may effect the conductiveity of that coil output. Should it effect it?

Also... what would cause this on only one coil output and not the rest?
Could the coil be at fault?

Thanks in advance
Dave