Why can't I mine something which resembles Crimtane ore?
Solution 1:
What you've got there is Crimstone, not Crimtane. You'll need a Nightmare or Deathbringer pick to mine that.
You could also use explosives, or Purification Powder will turn it into normal stone.
Solution 2:
Crimtane Ore, same as Demonite, can be mined with gold/platinum pickaxes. The Crimstone (entirely different from the ore) itself, requires a Nightmare pickaxe (demonite) or the Deathbringer Pickaxe, which is the one made from Crimtane ore.