Manual Removal - very carefully grab the bubble algae towards the base and use a gentle pulling and twisting motion to free it from the rock without breaking the bubble. Ventricaria ventricosa feels like large rubber marbles and they can burst thereby releasing thousands of new bubble algae cells into your tank. If they've already burst remove the bubble sack anyway. Don't let them get too big or you run the risk of them releasing the spores before you can remove them. Keep the upper hand here.