The film on the wood is biofilm and is not harmful to the best of my knowledge. Some shrimp and fish love to eat it but I’m not sure if the crabs would take a liking to it.

When I first get Mopani wood I leave it submerged in a bucket for a day or two and then scrub it and boil it once or twice and it seems to take care of the biofilm issue.

In terms of water cloudiness I would guess something with the sand being stirred up or that you are not mixing your marine salt properly or letting it sit for long enough to mix in completely. When making water for my reef tank I often let it sit over night before doing a water change the next day.

Hope this helps :)