I have an idea what it MIGHT be. THere is a little red spider, I think it is called spider mite in English (In Dutch: Spint). Is suchs the plants juices and leaves little scars. In the end, the colonies might kill a otherwise healthy plant. It typically occurs on 'unhappy' plants. So plants that stand in the shade when they prefer full sun, plants in a draft, or air which is too dry. Getting rid of it is fairly easy with a spray against the mites (Naturally, it is poison, and you need to deicde whether you want that on the plants.
Check the plant very carefully. The mites are less than a mm in size, reddish. Like the plant has been covered with some pepper.