Bring a medium pot of water to boil and blanch green beans until tender crisp, 2 to 3 minutes (don't overcook... you don't want limp green beans). Drain and put in a large heatproof bowl.
Bring 2½ cups of water to boil in another medium pot with vinegar, sugar, salt, chiles, peppercorns, and seaweed. As soon as the liquid boils, pour over beans and add the lemon slices. Let cool to room temperature. Slit 3 of the chiles to let seeds spill into liquid.
Chill, covered, overnight to let flavors develop. Serve cold.
Notes
I found the red chiles (Arbol) in my market's Latin products aisle, and I found the seaweed in the Asian foods aisle.
You can use regular, old black peppercorns if you'd like, but the multicolored ones are so pretty in this.
These green beans will keep covered in the refrigerator for about one week.