Generally---you WANT tiny life in your tank; a lot of it: a flashlight after dark should show you a LOT of critters on your sand.
Keepers: Good Guys bristleworms, spaghetti worms(in fact ANY worm, except one, listed below under bad guys). Spionids are fine. They're cleaners, like other things.
Mysis shrimp, tiny tiny tiny shrimp that make you think they're baby fish. Free fishfood.
Copepods and amphipods---copepods are white dots that move. Amphipods look like rolypolys or sow bugs. They do not have visible eyes! [if you see on of those, it's bad!] Again, free fishfood, and you can't have dragonets withOUT them.
Snails, stomatellas (saddle-shell snails). Chitons. Limpets. Wonderful guys. I'll add: strombus grazers, little snails that spin silk and use ropes to get where they're going.
Sponges (water filters). Occasional weird growths like networks on your rocks. These are great filters and improve water quality.
Occasional patches of algae that won't last long. In general, confine plants to your fuge. They block light, shed, and make problems.
Shrimp: cleaner and pistols, with caution: If you hear clicking in your tank---bad news. Pistol shrimp often kill fish. Pistol/goby pairs can end in the death of other fish in the tank. I had one kill its own roommate.
Micro-hermits with bitsy claws are fine. They walk on corals, but don't harm them. Neither would you, if you weighed that little.
Asterina stars: little starfish with a short leg: generally harmless Micro brittle stars, big brittle stars. All nice little cleaners.
Bad guys:but even these are ok in your sump I recommend AGAINST any crab but micro-hermits, whatever. Ever. Ever. Interesting to watch, but they need their own tank. Period. They eat fish, or other valuable things.
Shrimp: usually bad news, unless you want a mantis or pistol, and they're great specialty creatures: clicking in your rocks---pistol. Fish are in danger.
Eunicid worm: looks like a centipede with obvious tentacles on its head. Starfish in general. Green serpent stars.
Caulerpa algae: any rock that has it should be discarded.
Isopods: a cirolanid isopod looks like a roly-poly (sowbug) with obvious slanted black eyes. It attacks fish.
Flatworms--like the Star Trek emblem, a comet-thing on your glass or rock, with a forked tail.
Aiptasia, majanos---little 'volunteer' anemones, brown, nuisances.
Hydroids, look like a yellow-brown little mat of fuzzytopped sticks, about the diameter of a needle, about a quarter inch long. THey sting. Not nice.
In general, if you're in doubt of a hitchhiker, put it in your sump after posting a picture of it. Most things that don't go nicely in a tank can live a useful life in your sump, eating surplus food and detritus. A lot of neat things like barnacles, tunicates, and little clams don't last long in our tanks. Wish they did.
