Aiptasia diaphana (sea anemone)

Source: aquaduo.nl

QUICK STATS

• Care Level: Easy
  
• Reef Compatible: No, will overgrow
  
• Max. Size: 2 cm

Why the Glas Anemon is a plague, well, that's because they multiply very fast and sting very bad. You can inject boiling water in them. Use a needle and try to inject it in the "head" or "neck".
There are some fish and shrimps who have the glas anemon on their menu: Acreichthys tomentosus, Chelmon marginalis, Lysmata wurdimanni

Other links:
http://www.reeflex.net/tiere/961_Aiptasia_diaphana.htm

First one found in the crab tank, hopefully I can kill it before it becomes a plague.