Do you follow the manual’s terrible instructions to spray water displacement lubrication in the port at the end of every day of use?
If so, it’s just buildup mixed with the water going through the blower
Take the lid off your tank, crank up the machine and turn ip the rpm and run some fresh water through the blower, flushing out the contaminated buildup. Then put the lid back on, run it full throttle, full load and let it burn out any moisture. Once you’ve done that, turn it off and take off the lid to prevent any more moisture from building up. I’m sure you already know this, but some might not. Bawb taught me that process back in 2016 I think