No idea what this is, help ID please!

[LegalReefer]

This guy showed up on the glass when I got home today after a few days away, and I have no idea what it is. Looks like a baby jellyfish potentially but I have no idea. Help please?

 

[Gargoyle]

I was told that they were some sort of early stage jellyfish. But I'm not convinced. They go away after a while. Occasionally they pop back up after a really big water change but soon disappear again.

 

[GaryE]

Hydroid.. harmless and will go away on their own