Hi
I also had a little clear jelly fish like thing in my sump. I was excited to find something new, then it got ID'd for me and it was a flatworm !!
check this link about 1/2 way down to see if he's the same critter:
FlatwrmFAQ3
Mine feeds on copepods, which I need to feed my mandrin so I remove them when I see them, but I'm told they don't really do any harm, I guess.
Bonnie