Sounds like coraline algae starting to grow mine look exactly like snow flakes as they're developing. It'll turn a blue or purple once its grows probably. Its a good thing to have in your tank nothing to worry about.
I've heard they were feather duster larva. I actually had a feather duster in/on one so I tended to believe that. I always clean the front glass so I can see but leave the back to do whatever. Also, if it is in a snail line it could be snail eggs. As far as snowflake shape, if you can tell the shape and it's that big I would believe algae.
Let me know though cuz now I am interested to find out
I wouldn't say hydroids are harmless. Quite a few sting corals,fish & you.. I can/will take a tank over. But that is hydromedusae (Staurocladia oahuensis) AKA....Hydroid Jellyfish and generally nothing to worry about.