Tiny jellyfish, hydroid or feather duster clones? Photos included

[KM7]

What type of thing is this?
My nano tank is 4 months old now.
Suddenly after the Reef light went off there was suddenly 15-20 of them. Photo taken with normal lights.
I have a large population of Amphipods and some copepods.
Swims like a jelly, attaches to glass.
If they are harmless I hope they will stick around. I also have a feather duster in there I think and tiny bristle starfish

I will also include a photo of the feather duster - could it be feather duster clones?
Will the feather duster multiply if I only have one?

 

[Crabs McJones]

type of hydroid. they tend to go away as the tank matures