The first is probably another tube worm (and thus GOOD!) It's a different color than the others, having tricolored fronds--they have pale green tips, a hard white ring about halfway down, and then are blood red at the base. It's very pretty, for something incredibly tiny. I say "probably" because while the fronds look pretty tube-wormy, there's no visible tube, (although it could be buried in the rock, or the algae could just have grown over the rock, as it has to some of the other tubes) and they're sort of bifurcated. It's also FAST. The other tube worms pull in just barely fast enough to follow with the eye--this one is just poof! Gone. As invertebrates go, it's quite paranoid.
I have not yet slain the wily Aiptasia pest.
There's also something in the tank that I'm stumped on--it is red-orange and quite small. I first saw little white balls or nubs, very very tiny, on a patch of rock. They were very regularly sized, a little spaced out, (still covering maybe a quarter of an inch, nothing large) so I noticed them, but they were really small, so I didn't think much of it (and anyway, the rock is covered in specks of stuff.) Then I came back a little while later, and the little white balls had contracted together, and now had a small red-orange ring around them. A few minutes later, the balls were gone completely, and I realized they were the tentacles, or the tips of tentacles, for what is either a small red-orange polyp of some sort or a very very small anemone. (The two, contracted, look much the same.)
Either one is possible--on the assumption that bad things are more likely than good things, it could be a form of pest anemone called a majano. It doesn't look much like the photos at the moment, but I didn't get a good look while it was open. There are also things called ball anemones that it sort of resembles, but at the end of the day, I'm just gonna have to get a better look at it before I can get a positive ID.
And yes. I did just spend two hours looking at photos of corallimorphs on-line when I was supposed to be painting. Ganesh have mercy.
ETA: I have a ball anemone! Looks just like the photo, too. (The thing I was trying to ID may also be a ball anemone--still staring at it--but I found a second one on a different rock that could be a twin of the one in that photo.)
Depending on the site I read up, they are either a charming harmless hitchhiker or a fish-eating monster. Otter's had them for years and hasn't had a problem so far. Since it's not even a centimeter wide, fully extended, and all reports are that they reproduce very slowly, I have no particular fear for my future hypothetical fish at this time.
I am also told that that was some amazing live rock, to have this much stuff on it.