this is a common complaint on the USA website. It appears to be some sort of powder they use when assembling the cars. The American owners are having their carpets replaced under warranty where its not removable.
if its on the mats it is probably salt. The white powder i am talking about usually shows where the cars carpet joins or is trimmed round fixings ie round the seat fixing points, or under the dashboard. It seems to be some sort of powdered lubricant (talcum powder type stuff from the USA owners description). That the factory use or over use slightly.