can you identify what is causing this to my blockwork

[bulboy]

Hi, just got a semi-detached 1950ish with outbuilding. The north wall is almost all covered in this yellow-greenish stuff. it feels like powder when i run my finger and comes off without much effort.
Bear in min the photos are very close up. the reddish band on one of the pictures is actually the 10 mm standard mortar joint.
any advise is more than welcome.

Untitled

• bulboy
• 21 Feb 2012
• 1

Untitled

• bulboy
• 21 Feb 2012
• 1

 

[catlad]

It looks like a type of moss to me.

 

[noseall]

Lichens

 

[catlad]

That was the word I was looking for but could not spell it.

 

[^woody^]

North walls are invariably shaded, and promote algae and lichen growth

 

[bulboy]

Late update on this. Few years back and few years from post, took on this with the jet washer. All came down and is yet to grow back.