It WILL take over- I also don't believe when they say that it needs typically good water conditions to survive. I had it take over the LR in moy old tank right before and as it crashed. Then the rock because they had the nems on them continued to grow in the smaller holding tank. I mean I had probably 1,000 stalks.
It won't hurt your tank though, they are calcium based also that's why the are white at the base.
Jonny if you want I have some growing on the heater of the small tank, I can peel it off and bring it to you. I certainly dont want it but it you do it's all yours bro
What's strange is that I haven't added anything to this tank in months. I recently had a temp spike and crash then these things showed up after I got the tank going again