The pic at the top is an amphipod -- no problem having them around.
The star is one that I tend to move out our display tank and put down in the sump or refugium. I've not had it happen myself, but I've read claims that people have caught stars like that (with the blue / gray coloring) eating zoas. Again, not so sure how true that is (they could have just been eating dieing zoas), but better safe than sorry.
The flatworm is a little off color for what I would normally call a red flatworm, but it could still be. Red flatworms multiply like mad and can harm corals simply by congregating on them and blocking the light. That said, there's other types of flatworms that do not multiply so rapadily and do not pose much harm at all. IME, those are ones that have been more of a clear / white color. If the flatworms are indeed the dreaded red flatworms, there's a few fish (wrasses) that are likely to eat them, although they tend to only keep the population down if it's grown so large. Or, there's a reef safe chemical treatment for them called Flatworm Exit (made by Salifert). If you decide they need to go and choose the chemical route, please follow the directions, especially where it says to manually remove as many of them as possible before using the chemical treatment. The fluid inside the red flatworms is toxic and if the population is large enough, it's not hard to cause a bigger issue in the tank when most or all of them die from the treatment. Frequently, there really are more of them in the tank than anyone thinks.
