thank you for your question. what you are observing as filth is called smegma. It results from a collection of semen and other secretions that accumulate under the foreskin everyday.  As long as it is easily washed with water, does not smell or cause burning or pain its not a problem. However, if you have any of these symptoms it is best to get examined by a urologist/ andrologist who can help determine if everything is OK. hope this helps