A moving lump in your cat's belly may be a hair ball, feces, tumor, etc. You need to go to the hospital for a x-ray examination to determine the cause. Generally, cats need to be fed cat grass and hair removal cream regularly to prevent cats from getting hairballs. If they have hairballs, the cat will also vomit. If it is a tumor, it is best to treat it in time to avoid worsening of the condition and causing life-threatening consequences.