Hello,
I got this Bonsai tree a year ago and it has also a good place at the window + it has also good light and temperature. I am giving just the right amount of water and I am patient. But since a couple of weeks there are these white stains. What does it mean? What can I do?
I have attached the images and marked the stains.

Best regards

This tree do not get the right amount of water nor light. This is a struggeling jade tree close to death. More sun and less water is the key to save it, if the root rot has not gone to far. I give it 50/50 chanse of survival if the care is corected, 0 if not.

Your portulacaria has whoolly aphids. You need to remove them, often an insecticide is used. Make sure you ask in the store whether the product they recommend is safe for use on succulents.

Next to this, this plant is in desperate need of some light. Get is some sun. It is a desert dweller, which loves heat sun and -when available- water.

Indeed the tree have wolly aphids. But they did not cause the unhealthy tree. They just came there to feast on a weak tree.