that patio is very bad , not cleaned in years if ever . thats a great washer sdrah but looking at your pics i reckon that powerwashing alone wont shift the lichens etc , i normally treat after with hypo and they come up spotless . another possible problem will be the mortar joints , if any are damaged inform the customer before you start and you cant be blamed .also if your using a turbo nozzle on them it can easily blast out the mortar so maybe try an ordinary nozzle before you could do damage