Chaeto does not actually attach to the rock, even after a long period of time, ime. Just keep pruning it back and change elastics once in awhile. However, I have had bits of chaeto, that must have detached from the main "chaeto ball" while pruning, attach to the backs of my tanks. Thought that was kind of weird since it has never attached to the rock it's elasticked to. (Don't know if elasticked is even a word
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I have chaeto and other non-invasive macroalgae in higher flow areas of all our no-skimmer, no sump/refugium, BB tanks. I also try really hard to keep a low bioload and not overfeed.