My pots are turning white outside because of the mold and it bothers me a lot. Is there anything I can do? Does it have to do with the humidity of the place where I live (Portugal) or with the type of substrate I use? My plants look good and healthy. I tried to wash it, but the mold always comes back.
This is perfectly normal for terracotta pots - they are porous, so deposits from hard water, fertilizers and so on seep through to the outside and, once dry, are noticeable. Believe it or not,some people like this 'aged' look and actually apply yoghurt or other dairy based products to encourage deposit formation on new pots.
You can scrub it off periodically if you really don't like it, see here https://homeguides.sfgate.com/white-powder-terracotta-pots-91471.html but it will always build up again. Glazed ceramic and plastic pots do not have this problem.