Sounds like a leech to me. I had them in one of my tanks for a while and they definitely lived in the substrate. Many types of leeches are harmless detrivores, but even so they grossed me out. Interestingly, they finally disappeared after I introduced blackworms to the tank.
Don't feel bad about flushing it too quickly though. I'm confident you'll get more shots at making an id.
Life isn't stone, it's water.
PSA: When talking about baby fish, "fry" is plural.
I have a fry. I have some fry.