killers?

my friend has a mix african cichlid tank, and just today he was cleaning the sump when he noticed these small creatures swimming around at the bottom of the sump. the are too small to take photos of but big enough to see them as they move around(trust me i have tried many times ). what could they be? do you think they could harm the fish in any way?
thnaks in advance.

they are white and are on the glass. they are very small and thin. he feed the tetra doremin. thank for the reply