Red mould on external rendering

[tufty1]

Hi folks, first timer but reading posts for ages, great site and hope to be helpful to others to.

I have an existing old stone shed which i'm currently converting into a bedroom. However, the external rendering, which is painted white, has patches of a reddish sort of mould on it and i'd love to know whats causing it and how to get rid, any ideas? Apart from being un-sightly i'm drylining the internal walls and hope i'm not going to make it any worse, may have to put in air vents to the 'cavity'.

I'm thinking its a typical sand/cement render which is'nt allowing the moisture in the stone walls to evaporate, causing the red mould! I'm sure the walls don't have any dpc and i expect if i paint it, the red mould will just come back. Thanks alot for any replies.

 

[MOB]

I think it is some class of lichen. I think it is a sign of a clean environment.

 

[VT1]

I have the same red discolouration in circular patches on a recently skimmed new ceiling. would love to know what it is. it cleans off with a cloth and the plasterbard / ceiling does not feel damp.

never seen it before. wondering if I repaint will it come through again

VT

 

[MOB]

OP has the red on external render. Your ceiling is unlikely to be the same thing.