This is a cyanobacteria, one of the so called 'blue-green algae'.  You can usually see them when they’re concentrated into brownish or greenish clumps, often appearing as a scum, but which can sometimes look quite 'plant like'.

In fresh waters they can be found suspended within the water, but are often attached to rocks and other damp surfaces where they form large clumps.

Water born algae are normally encountered in the hot summer months, but clumps of algae in damp areas can be seen in most months of the year.

Common

Common