Strange spot on the wall - what could it be?

Hello everyone,

this concerns a single-family house built in 2004/2005; at one spot on the wall plaster, strange deposits are visible, see photo. According to the documents, a "Buntsteinputz No. 900 with a grain size of 3 mm" is applied in the base area, and on the house wall a "Terrastar 222 disc plaster light 3 mm". Around the house there is a 40 cm wide eaves strip filled with gravel. However, in front of it are flower beds, many hydrangeas and other larger plants, some of which partly cover the eaves edge and then also partly reach almost up to the house wall. The stain ranged from gray to brown to slightly pink.

At first, because of the pink coloring, I thought of algae caused by the shading of the house wall at this spot (the wall faces exactly north). However, at all other places on the same wall these stains do not appear, so it cannot be due to the north orientation alone. Also, there is no vegetation exactly at this spot. My second guess was that previous users had permanently installed a hose system for watering the flower beds and that (quite iron-rich!) well water was sprayed uncontrollably onto the wall from there for a longer time. This could also explain the shape of the stain: to me it looks like stalagmites growing upward from the floor of a cave. A watering system could have sprayed several thin water jets onto always the same points of the wall, from where the water then ran downward in a widening trail.

Yesterday I vigorously brushed Biostein wet into the wall with a brush (for those who do not know Biostein: a very good household cleaner, removes most things), let it act for a quarter of an hour and then scrubbed. A strong ocher-yellow brew ran down (which unfortunately I did not photograph in my enthusiasm for scrubbing). The last photo shows the scrubbed wall, not yet completely dried again, the remaining stains do not come off further.

What do you think about the algae theory? What do you think about the theory with the permanently installed watering system? What other causes for this damage pattern come to your mind? And: Which of these is most likely, and why?

Many thanks and best regards,
Marco

my first thought, rising moisture, after scrubbing the thought remains. The moisture barrier under the exterior wall could be damaged. I would get a professional now, as long as the damage is still manageable. Walk around the house to see if this is the only spot. Which direction, windward side?

Now the question here?

Here, too, the answer will be that it looks like usual dirt and algae buildup. No rust is visible.
Rising moisture if in doubt as well.

my first thought, rising damp,

Does it fit that the other side of the wall, around the corner to the right (clearly visible in photos 1 and 2), is completely intact? Shouldn't it also be damp and efflorescing there? The wall faces exactly north.

Here too, the answer will be that it looks like usual dirt and algae buildup.

Only this one single spot on the whole house? Everywhere else just the usual dust deposits? Does that make sense?

North and behind plants is predestined for this. It gets wet again and again and hardly dries because no sun reaches it. You can observe this in every new housing development.