Translation:The meal is made of potato and water.
My mom told me that's how her family in Portugal used to eat, because they were very poor.
That's probably best, but I don't think "potato" in this sentence is necessarily wrong. At least one well-respected on-line dictionary includes a definition of "potato" as a mass noun meaning mashed potatoes:
You probably don't believe leek and potato soup is made with a single potato.
Yeah, I think in British English it's more common to say 'potato' as a mass noun than in American. I believe they say mashed potatoes, and we say mashed potato, for instance!
This not something one would say in English (in the UK anyway). We would say 'the meal is made up of potato and water' which was marked wrong.