Condensing boilers are great, but the pH of the exhaust can be pretty low. Low enough to dissolve the gasket that came with the boiler. So Triangle, the manufacturer, came out with a replacement kit with a gasket that is much tougher than the stock HDPE o-ring it came with.
What you are looking at is the remains of this replacement gasket in the cleanout for the boiler's drain line.
Now Triangle has a new gasket kit with a supposedly even better gasket. Let's hope I'm $28 from a correctly working boiler.
Does this matter? A little. The exhaust is corrosive, so when the gasket's gone there is always a little corrosive, humid air leaking into the basement. Not very much, but the gasket's there in the first place because your flue isn't supposed to leak into your house.
Oh, and carbon monoxide, but not much of that on a condensing boiler, and I have a CO sensor in the basement. So far, no detectable CO.
I think anybody with gas-fired space heat or water heater should have a CO sensor. They're cheap, so there's no reason not to.
It also doesn't hurt to look over all your major systems fairly thoroughly once a year. I caught this one pretty much by accident because my father-in-law and I were talking furnaces and one thing led to another. I have a list of annual maintenance items. Checking the boiler cleanout for signs of gasket degradation is now on the list.