Riccia is a real mess maker!
You can try taking out the wood and trimming, or you can trim it in the aquarium and skim out the trimmings when you are done. I do the skim thing. It takes me almost 45 minutes sometimes
Take a good look at the Riccia tied to the wood. There is going to be a point where trimming isn't going to do it, you will have to retie it.