My guess, could be earthernware that sat on silica sand in the kiln, some happened to migrate into the pores?
Totally confused on what this could be. That is my best/worst guess. Never heard of any clay body dropping small sandy particles.
Similar thought/speculation. Maybe the potter has mixed alumina into their wax and it migrated into the pores? This can leave a dusty fine residue on the pot. If its this, the pot should be fine once all the alumina is cleaned up.
+1 for Neil's wet sanding suggestion.