I emailed B&C to check. Got the following reply, so doesnt seem to be any reason to worry:
It is normal.
What you see is a white residue due to the evaporation of the adhesive's solvent used to glue together the phase plug components.
This behavior does not affect absolutely the performance of the phase plug or its reliability, but it concerns only its cosmetic appearance.
Unfortunately, once the phase plug is assembled, the parts covered by the white residues are difficult to reach. Their removing would be too much time consuming, so they are left on the parts.