Dirty coil

I've just replaced my sparks with Denso Iridiums,and noticed one of the coil packs was filthy (the one nearest the battery). Its got a rust coloured coating from the bottom to about half way up - it rubs off ok,but it's also inside. It seemed to be done up as tight as the others.

The others look like new!

Any ideas - could it be on its way out?

No misfires or anything.