You can say they are good because they will eat nutrients in your tank, but most of us consider them bad because:
1.) they multiply fast
2.) are ugly
3.) will sting other corals
Basically as stated above, they can take over.
There are many methods to kill them, personally I just mix KALK with water to make a pasty liquid and then using a syringe, I inject and cover each one. Will usually kill them in one try.
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Setup: 180G DT, 105G Refuge (approx. 300lbs LR, 150lbs Aragonite)
Hardware: Super Reef Octopus SSS-3000, Tunze ATO, Mag 18 return, 2x MP40W, 2X Koralia 4's Wavemaker
Lighting: 5ft Hamilton Belize Sun (2x250W MH, 2X80W T5HO)
Type of Aquarium: mixed reef (SPS & LPS) with fish
Dosing: Mg, Ca, Alk
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