playing the board.. do the cards speak?

[lighterjobs]

$6-$12 hold'em game. I hold 66 on the button and there are two limpers. I raise and BB calls as do both early limpers. Flop comes 345 rainbow. check, check I bet. BB folds and two calls. turn (345)2. mp1 bets, fold and I raise, mp1 calls. river (3452)6. mp1 checks and I turn over my straight on the board. the mp1 says "i'm playing the board" and mucks his cards. what should the floor do?

edit- I meant to put that the other player thought I just had a set of sixes, where he had a pair and dind't see the straight on the board and tossed his cards.