According to the website of the company that manufactures Smelly Washer Cleaner, the product does not harm machines. It is also safe for septic systems. Online customer reviews of the product - even negative ones - mention nothing about machine damage.
Smelly Washer Cleaner is an all-natural powder designed to remove musty odors from washing machines and laundry. A capful of the product in the hot cycle cleans any front- or top-loading machine. While most are satisfied with the product, some consumers report that it was not as effective as promised or that the odor elimination was temporary. They did not report damage to their machines.