What to clean Portland stone with??

[Marley Farley]

Hi all, Anybody got any ideas what I should clean Portland stone with  It has lichens of all colours & types growing on three sides, which are lovely & I want to keep of course, but I have this green algae type stuff on the other side where plants have been up close to it I would like to keep this side of the stone clean. I have been told not to use bleach or the like as it is quite well fossilised & that would damage it.   Have used soap & water so far but it doesn't really get it off. So any help appreciated, thanks 

[Tulipa]

My husband has taken apart a rockery wall and is rebuilding it.  He has been using a cheap pressure hose not a very powerful one and it has been just enough to remove the algae and leave the lichens.  I don't know if you have enough stone to warrant the expenditure, we have 100ft wall so £40 was worth it as it is taking weeks and we couldn't have kept hiring one each weekend.

[waggi]

how big is the rock

[Marley Farley]

It's about 2' high & the side that the algae is on doesn't get much sun I have no power or running water there either so any ideas thanks 

[waggi]

scrubbing brush and water then i think
all washed up there sorry

[ACE]

Bleach, watered down and paintbrushed on at night, hosed off the next morning, repeat until clean.( Used to clean headstones) Works well on purbeck, portland and granite but do not use it on marble.