Question

Why does algae become black when we touch it?

Answer 1

There can be many answers to this:
for instance the part of algae that you touched might just die so it may become black after some time.
Or the algae was on that spot for quite some time, and after some time the blue-green algae becomes black because it hardens its position at that place, and becomes more powerful, and it becomes much harder to remove the black algae.