Inspecting an R/C Airplane Glow plug

I have a Jett .40 (424) motor and have been using the same plug for a while. Several times the motor quit midway through the flight and after thoroughly checking the fuel system I changed the glow plug and the problem was cured.

I carefully inspected the plug and the only thing that was unusual was the coil had a frosted look rather than a shinny smooth finish. Can anyone explain what causes this and when it happens should the plug be discarded?

Thanks for any help.