"May I help you clean the walls?" should be accepted. It is more acceptable grammar in English than to say "Can I help you clean the walls?" "May I" and "Can I" are often used interchangeably but they are not equivalent. Unless the question is asking if I am "capable" of helping you clean the walls, it absolutely should be "may I" -- meaning "will you allow me to help you clean the walls" or "is it okay if I help you clean the walls."
In english, "can I" means "am I actually able to/ capable of doing" something. We are always taught to say "may I" which is a question of desire to do something. As in may I help. In general speech either would be OK but the latter in this instance is gramatically correct.