Can't remove crank arm on older Mercier
What you are looking at is called a "Cottered Crank". The shaped cotter pin holds the crank in place and they can be next to impossible to remove without significant effort or a cotter pin press.

Sheldon Brown has an excellent article on removing cottered cranks using a hammer and pipe, which is a method I have used before, albeit multiple decades ago.

When you get it off you may want to consider replacing the bottom bracket and cranks with a more modern design that is easier to maintain, unless this is a restoration job and you want to stay true to the original.