These tiny green anemones seemed so innocent. I bought four just for their color, not knowing these were Anemonia Manjanos, which quickly split and made more of themselves and spread. Soon I had 40 of these guys in my tank. They are attracted to light, and will climb the rockwork to get to the top. Their sting is damaging to corals, and eat up 'real estate' other corals could use. This is definitely a pest anemone, and using a syringe with boiling water, I removed every one of them.
Here's a couple more images since their appearance varies somewhat based on lighting.