Tell the well man your casing was never cemented in. He will probably flip out, but he will know what you mean. That space around the casing should be sealed with cement. You can't just pour it in or it will also fill the well up. Depending on how wells are done in your area, it either needs to be gravel packed, or packed off to have a base for the cement. Bugs and everything else can get in your well.
The muddy water is probably from a leaking fitting on the new pressure tank. The water is leaking into your well head, and that is what's making the water muddy. But the water is not coming over the top of the casing like the well man thinks, it is coming from the plumbing and running down the bore hole.