Array
(
    [0] => Array
        (
            [mrt_id] => 4
            [mr_agent] => 1
            [mr_contragent] => 4
            [mr_status] => 1
        )

    [1] => Array
        (
            [mrt_id] => 6
            [mr_agent] => 1
            [mr_contragent] => 6
            [mr_status] => 1
        )

)

Array
(
    [0] => Array
        (
            [mrt_id] => 1
            [mr_agent] => 1
            [mr_contragent] => 4
            [mr_status] => 1
        )

    [1] => Array
        (
            [mrt_id] => 2
            [mr_agent] => 1
            [mr_contragent] => 4
            [mr_status] => 1
        )

    [2] => Array
        (
            [mrt_id] => 3
            [mr_agent] => 1
            [mr_contragent] => 4
            [mr_status] => 1
        )

    [3] => Array
        (
            [mrt_id] => 4
            [mr_agent] => 1
            [mr_contragent] => 4
            [mr_status] => 1
        )

    [4] => Array
        (
            [mrt_id] => 1
            [mr_agent] => 1
            [mr_contragent] => 6
            [mr_status] => 1
        )

)

$result = array_diff(первый, второй);

Array to string conversion