urgent Yamaha SHO question - prop exhaust build up

I have a 250 sho and never noticed exhaust build up in my prop. last time I checked for sure was a few weeks ago when I removed prop tocheck for fishing line, prop was clean and castle nut was clean.

Today we did a lot of idling and a lot of low speed planed runs but did open it up to WOT at the end of the day for 10 miles. After pulling boat out the inside of prop and castle nut is covered in soot. It isn't horribly filthy like an old carbed motor but it's unusual for my prop.

I just had 100 hour service 10 hours back and everything was good

Does this sound normal given how I ran it today or does it sound like something to get checked out right away? The motor ran just fine today and when wide open it hit full rpm and speed

And normally I don't idle and troll a lot so if this is normal given how I drove today that would explain why it was clean in the past

Not a problem

It sounds like maybe a thermostat or two is stuck open and letting the motor run too cold. We haven't seen any SHO with a build up of carbon / soot! I would keep an eye on it.