Algae
My Grandfather was a master decorator and swore by a vinegar, water and washing soda mixture for washing (sometimes the algae gets "into" the mortar/plaster), then, for outside walls (to "seal" them) mix cow manure with water, paint it on, allow to dry thoroughly and paint over it, seriously, no smell !
For inside walls, seal them with egg white (painted on after washing) and you can then wallpaper or paint without any worries (works for dampness also).
Hope this has not disgusted you, it works. Sometimes the old tricks are the best, without chemicals.
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