Make Clay reset at the beginning of your next turn.
This way Dim Shields will time out regardless of how many times they are Morphed into.
E.g. If you morph your own shield, you'll lose all trace of the original shield one turn later and be unable to morph it again. If you target an opponent's, you can only do so while it is in play. Once it's gone it's gone.
In that case, you may wish to make permanent abilities available immediately (or not, as a versatility nerf).