Moss can cause problems. Loose feeders just get lost in it or die and mold over. It's hard to keep fecal/urate matter from building up in the strands. Chams can pick up particles on their tongues and ingest it...can result in intestinal damage or blockages. When constantly wet it does harbor molds and bacteria. Some types of moss have sharp spines that can injure tongue, gums, and intestines.