xenias and such aren't as bad as long as you place them closer to the top of a rock as opposed to giving them the opportunity to grow outwards...if you do that then they surely will overtake whatever they can...if they are close enough to another rock or your glass they might attach to that as well so be careful placing them too close to other things
once they grow on something it's almost impossible to kill them...you can try your best to pull them off but any residue left will grow and multiply in a matter of weeks