steve wrote:Not sure. It could be fungus but it does not come off easily. Have you tried cleaning this off?
I just have tried cleaning it off, the big white cover (2nd and 3rd close-up pictures) has disappeared, so maybe it was just dirt?
On the other hand, and this is the weird part, the white spots (1st close-up picture), are MOVING through the shell when I touch them with my finger

. Not all of the spots are moving, but most of them are. So... I just figured out that maybe they are air bubbles trapped between the shell and the shedding layer? Some of the spots disappeared off the shell's border when I pushed them out, other ones, however, don't move at all. I'm really astonished about this

Maybe the unmoving remaining spots are not bubbles but fungus?
In regards to your previous answer, I didn't completely understand you, you meant that fungus should come off easily or not?