Those are a small feather duster that grow by matting across space. They will grow in between your zoos. Maybe to the point where you can't seperate them.
I had these in my frag tank. I got them from a maricultured coral I bought from my LFS. I thought they were kinda cool at the time. Over about 6 months, they grew everywhere in the frag tank. They even grew in the sand.
It took quite an effort to get them out of my system. Some are so small that when I cleaned them out, I fought them for a good 2-3 months after.
So, question is... do you like them and can you manage them and keep them seperate from other things? If not, get rid of them now.
Definitely brown hydroids. These things can be worse than aiptasia if they get started. Kill them all now when you can. Use a pair of Sharp tweezers and pluck what you can. If anything remains cover it with superglue then rip the superglue off when it dries thoroughly. You only have a few so they will be easy to exterminate if you do so quickly and aggressively.