Hello Hendrik - oh no! Sorry to say bud - you have the dreaded aptaisia anemone...
These things spread LIKE WILDFIRE! They don't do anything specifically to the tank or inhabitants, except for take over completely. THEN - once they have infiltrated between corals, etc, they sting the corals and the corals don't come out anymore :-(
To get rid of them - THAT's ANOTHER STORY.... People suggest vinegar injections, kalkwasser injections, "Joe's Juice" injections, lemon-juice injections, etc...
I have tried all - but to no avail.... So - you too can try the above suggestions, and let us know IF any of them work for you.
Um, just in case you or anyone else reading this didn't realise triffid anemones and aiptasia are the same thing. Aka "glass anemone" as well. Aka "WHYYYYYYYYYYYYYYYY must this happend to ME!!!!!!!!???????????? Why won't they just go AWAY!!" and sometimes "cute little anemone on my live rock, score!"
Hi. Sorry I am only replying now.
I appreciate all the adivce.
I have borrowed something (can't remember the name) to inject them with.
They do however seem too small to do it now, so I will wait for them to grow a little bit more.