No, never use bleach in a power washer. Bleach is an extremely effective chemical which can be used to clean moss and other stains as well. Keeping this in mind you should never use it in a pressure washer as it can corrode your pump and any other metal in the spray. This would then render the washer useless as it won't work. Besides this, inhaling bleach can be even more dangerous to your health and wellbeing. But how would bleach be inhaled you might ask. As you pressure wash your home, you would be propelling it into the air as well along with the spray of water. Doing this you would stand a chance to not only harm yourself but also any plants or vehicle the bleach would come in contact with. Moreover, If you really have to use bleach. Then scrub it onto the place manually and rinse it out before using the power pressure washer.