I believe that the egg mass will be from the blood worm fly.
It looks a bit like a mossie, but doesn't bite.
The egg raft of mosquito is about four mm long, two mm wide, dark brown, floats on top and is bent like a banana, hence the name 'raft'.
If squeezed between the fingers in water, it will break into 100's of tiny segments, the eggs.
I use the rafts in newly ,well, a few days old, killies,
it is like a meal on the hoof for them.
They change from a wriggler, having only one breathing tube,
from the end of its body, to a 'tumbler',
breathing from two tubes of its head.
From this state it backs up to the surface, s
plits its back open and emerges as an adult mossie.
It uses its old body shell as a float as its wings dry,
then when ready off in search of a feed of blood,
which it must have prior to it having the ability to lay more eggs.
One good thing tho, only half the mossies bite and suck blood.
Wouldn't you know it ! !
Yes,
It's the female.
Alan