[s-cars] White stuff in my exhaust

[Michael DeBlasio]

Tonight I pulled my turbo and EM only to find some white stuff caked onto
the inside of my EM (only near outlet to the turbo),  the inside of the hot
side of my turbo, all over the fins, and beginning part of my down pipe.  It
looks like powder, but it is caked on like calcium, in fact is appears to be
very much like calcium.

My questions are:

What is this stuff?

Why is it there?

It is coating my O2 sensor, so will it affect how well it works?

TIA,

Michael
'93 S4