Its really just a matter of aiming the pickax not the cross hairs in the right direction. In the sealed up stone walls you sometimes have to jump onto one of the blocks if you can and again aim the pickax not the cross hairs. This has worked for me clear each Ancient Crypt of any mud piles.

Just move on. All those hits on unbreakable material by your pickaxe still use up its durability, so you're really just wasting a lot of time and effort for something that may not even give you scrap iron.

Jump + pick
If u cant mine it , just let it there , cant be mined

There is no better way. Just move on. Your game won't be affected because you MIGHT miss 1 or 2 of scrap irons in a crypt. Better use that time to find another sunken crypt.

Yeah I just do a running jump. Swing your pickaxe immediately at the start of your leap so that it connects as you reach the wall. I'm usually able to reach everything, except for this one really tucked away piece near the top left.