Are frozen mussels/clams supposed to be slightly open?
You are right to be concerned.  The main reasons for frozen bivalves to be slightly open: 


They were steamed before freezing.  This is common, and fine.  You should be able to verify this by thawing 1-2 of them and checking if they are cooked.
They were frozen very slowly.  This is bad, because the bivalves can die and go bad during the freezing process.
They've thawed and then been re-frozen.  This is also bad, because it also makes contamination much more likely.


If you check them, and it turns out that they are not precooked, then you are taking a substantial risk by eating them.  I wouldn't do it.

Sorry that I have no references for the above; I've searched the relevant literature, and the only thing I can find is some Reddit questions verifying that it's normal for bivalves to open on thawing.