something wrong

I have just started a game and there are pieces that have already been moved, he has called the game " SLAV" should I play or report him

Did the seek specify Slav? Weren't the pieces moved before you started? If there is less than 4 moves made you can resign, no harm to your rating.

You've accepted a "Thematic" game where the position starts at a certain position, to ensure that a certain opening is played.

Sometimes, this feature can be abused to start a game where one side has a clear advantage, but that one isn't one of them.  If you want to try the Slav (a branch of Queen's Pawn openings), then play on.  If not, then don't (You should be able to ABORT the game with no repercussions).

Don't report him, however.  It's likely you just didn't realize that the game was a Thematic one when you accepted the challenge.  I doubt there was any nefarious intent.  You opponent most likely just wanted to practice that particular opening.

Thanks I understand now

Start New Game, Standard Opening, or an Opening of the Settings you choose are both options, any ECO or Standard are options / features of this site.  No need to report anyone, it is a game to be played in a particular opening.  The same goes for choosing ratings to have a game against, all is fine.