I have lots of these attached to the glass of my tank. They are about 1mm in diameter. I couldn't take a picture of them because they are so small. Here's a drawing. They are white and have 6 legs.
NOT Copepods but Hydroids
looking like jellyfish, they can sting in numbers. Hard to kill but can be either pulled with tweezers or drop a dot of gel superglue over their center
Yes, hydroids - medusa, but the ones that show up on the glass like that are very temporary.
Nothing to break out the super glue for, IMO. When I dose live phyto, grow a bunch of baby copepods, these appear on the glass.