Welcome to AA!
Yes, it sounds like you've injured it. Remove the rubberband very carefully, and it may or may not recover, depending on how badly it's damaged. 'shrooms
are pretty tough.
In the future, rather than banding it, place it in a small dish filled with rubble or sand, use a netting to cover the dish and let it sit for a while in your tank in low-flow. It will naturally attach itself to the rubble, then you can glue/epoxy the rubble where you want it.