There are many ways to describe a crab that plays baseball. You might call it a "pinch hitter" which means that the crab is called to hit in place of another player at the moment they are unable to play. Another possibility is to call it a "baseball cracker" which suggests that it might have a playful or mischievous personality similar to a baseball player. Additionally, you could simply call it a "baseball crab" to describe its activity.