Styrodur perimeter insulation - capillary effect on the reinforcement mortar?

Hello,
I have skimmed the exposed Styrodur of the perimeter insulation with reinforcing mortar for UV protection. Now the following picture appears (after prolonged rain) with band-shaped moisture.

Does this come from splash water or is it being soaked from below?
The latter would be very unfavorable.

As far as I remember, it is
SAKRET adhesive and reinforcing filler
and the product description states
for indoor and outdoor
water repellent

Problem: A conservatory with a wooden substructure is still to be attached. Rising moisture would be deadly. :-(

Anyone have an idea?

How can you remove this stuff again without damaging the surface underneath?

What would be suitable instead for filling/plastering/cladding?