Christians, muslims, and other mythical creatures

My mother is a very religious person, and she likes to go to mass every day. We have a church within walking distance of our home (it's actually the church that Alfie and I got married in), so when she visits us she likes to walk to and from the church for daily mass. Yesterday was a rainy day, so I offered to drive her and my dad after we picked up 3Po and Jammy from school. After we dropped them off, 3Po and I had an interesting theological discussion:

3Po: Lola (that's what he calls my mother, it's the Filipino word for "grandmother") is a Christian, right? Do Christians believe in God?

Me: Yes.

3Po: What about Muslims?

Me: Muslims also believe in God, but they have a different name for God -- they call him Allah.

3Po: So what do you call a person who believes in mermaids?