The derelict area in which these colonies were found is the foreground that is shown covered with snow in the following photo: NS3977 : Mounds of rubble (on site of former walled garden)
The seaweed-like mass shown here measured 15 cm across. It is a colony of cyanobacteria; specifically, it is Nostoc commune. For more on cyanobacteria, see NS3977 : Cyanobacteria - Nostoc pruniforme
, taken on the same day, in the same place; they are bacteria, but, unlike most bacteria, they perform photosynthesis (hence the green component of their colour).
Within its outer membrane, the colony contains a brownish gel, forming a matrix in which chains of green photosynthetic cells are suspended. The colonies vary greatly in appearance according to how wet they are, but are generally olive-green to brown.
Although I've shown a single specimen here, the area was covered in seaweed-like patches, up to several square metres in size. Two factors account for the abundance: (1) for the west of Scotland, the summer of 2009 was the second-wettest since records began in 1914, and (2) there is great deal of concrete here, from large chunks to tiny fragments (the Nostoc pruniforme link, above, explains why this favours their growth).
The abundance of N. commune was not confined to this spot; later on the same day, I noticed that a little concrete NS3977 : Footbridge over the Murroch Burn
in Kilmalid was also densely covered: see NS3977 : Nostoc commune on footbridge
, taken on a later occasion. Incidentally, when they dry out, Nostoc colonies become extremely inconspicuous.