Spray with water and soak. I apply chems through a pump sprayer (Virosol, Ubik, No nonsense Degreaser or Dave Kemp TFR) whichever you prefer. I am trying them all at the moment, not sure yet.
Agitate, leave for a couple of minutes then scrub off.
If all else fails, then a weak hypo mix should fix it. I never spray this, just mix in a bucket and apply with a brush...an old brush!
Any of the above chems should do a good job. Leave the Hypo until you have tried the others.
PS don't pressure washer them.