My succulent has developed some weird disease, first it only looked as if something was eating it but then the leaves started turning pale and yellow.. I never fertilize it and I irrigate it once a week. Does anyone has an idea what the disease is about and how to treat it? second angle
The problem does look likely to be environmental rather than an infestation or disease. Jade plants (Crassula argentea) like plenty of sunlight, particularly in winter, and good air flow around them, so if it's in the house, near an open window in summer is helpful. You haven't said where you're keeping your plants but I think that's a Haworthia I can see in the background, off to the left, which appears to be in the same container. Haworthia do not like direct sun, they just need strong daylight, so the two plants are somewhat incompatible in terms of light requirements. They like average temperatures, but much prefer a good differentiation between day and night temperatures, rather than its being even all the time. It doesn't seem as if you're overwatering particularly, but all succulents should have reduced watering in winter, only sparingly watered, and once a week may be too often if it's winter where you are.
At this stage, I'd suggest you take it out of the container it's currently in and pot it up separately, giving it the conditions it requires, to see if it recovers and grows normally.