I think it's 'Murican. As in 'Murica, which is a play on the colloquial pronunciation of "America."
I wasn't aware a Mercan was a thing. Can't imagine why anyone would use one.
it's a regional thing on how it's pronounced. like how some people say "Warshington" or "bah-trees" or play Duck, Duck, Grey Duck instead of Duck, Duck, Goose. in Hhwhyomin, it was 'Merca. in South Dakota, it was Uhhmerca. here in rural Minnesohhta, it's pronounced "Vikings" or "Twins" depending on what time of year it is.