New Cat, Odd deposits on lambda?

Ok so task for today was to fit a new cat for the MOT

We had a emissions check a few months back after i thaught the exhaust smelt very fuelly, and the results showed the exhaust was at lambda 1, but the CO readings were well above what was allowed.

I simply assumed the 12year old cat had died so ordered up a replacement.

Came to fit it today and after finally getting the old one out, it turns out the insides have been gutted. The bolts from cat to downpipe were rusted solid and corroded away so its been on there like that for a long time too!

What i did notice however was a white dusty looking coating over the tip of the lambda sensor and the inlet to the (now gutted) cat box.

Any gen on what this white dusty coating is? My first thaughts were silicone contamination, but the fact that the emissions test we did resulted in a reading of lambda 1 would suggest the sensor is still working...

There also hasnt been any fault codes recorded for the lambda..

The sensor has been replaced at some point, it looks shiney and new on the outside, and the cable is nice and clean compared with the rest of the engine.

We'll see if it passes the MOT, if so i'll happily ignore it