discoloration

Hey friends, I have a 1978 Ericson 27' that I put on the hard this year. After taking a look at the bottom I noticed some discoloration around the drive shaft on the exterior of the boat. I examined the inside and it' pretty dirty where the drive exits the engine and goes through the hull but...

Initial question from me: I change my transmission fluid every year (Hurth HBW50 tranny, I think, on an M25XP), and in between I put around 20 hours of motoring in. Alignment is reasonably good, and I have a flex coupling. No strange gear noises or slippage and shifting is very easy...