I got the same crust on my PSS, I think its just salt and calcium, I cleaned it off the seal when I had the thing apart and I now see it has returned.
When I cleaned it off it was defiantly foreign and nothing to do with the seal. I too have no leaks from the seal so it must be just secreting from the film of water that lubricates the seal face.
I wouldn't worry about it,,, just carefully chip it off and give it a wipe

It's actually from electrolysis!!

Don't ask me how or why I just have spoken to an expert and that what he said!!!

Carbon material is semi conductive, and it will form an electrolytic cell with metal. For example, carbon & ali composite in a very wet area is generally a no no.

That's a good article. On my yacht I have one shaft anode. On my old fishing vessel, Oregon on hardwood frames I used one anode to connect bronze rudder to shaft/prop. Bronze keel cooling and all the skin fittings had nothing. Now 72 years old [I don't own her now] original fastenings still good. No anodes is better than too many.
Couple of weeks ago I was at the slips and there was a 27 foot fibreglass yacht on the slips. It had a lead keel and the owner was just tightening up the bolts as I arrived. He had attached a trawler anode 6 x 4 inches to the bottom of the keel at the trailing edge! Well over a metre from the prop which could have done with a shaft anode. I tried to explain what anodes are for but it was like pissing into the wind!

Yes, thanks rumble. I just rang D H Porter about the crusty stuff ( I bought the seal from them ) the
guy I spoke to said he had no idea what it was, so he was no help. I'm glad I'm not the only one with
it (jolene). It would be handy if others who have it reported in so we can see if it's a wide spread
problem or if it's just affecting me and Jolene.