We put expensive composite decking on our house which was supposed to be no maintenance and I have crap growing on mine also. We use a power washer and it does a really good job...still sucks as it was supposed to be no maintenance.
That would be disappointing. We looked at that composite when we rebuilt the deck in back and decided "nope, not this time". We used wood and painted it with that super-thick stuff that's supposed to be almost indestructible, fill in nail holes, etc. Um, not so much!