Answer

The correct spelling is usually:

Michhami Dukkadam

Another common spelling is:

Micchāmi Dukkaḍaṃ

So, “michhami dukkdam” is understandable, but it is better to write “Michhami Dukkadam”.

Meaning

Michhami Dukkadam means:

“May all the wrongs I have done be forgiven.”

It is especially said during Samvatsari and Paryushan as a humble request for forgiveness from all living beings.