I have grey lines too, and don't know what they're from! I can't tell from your pic if they are the same as mine or not. My pic is from a bean that is covered in them, but I also have many beans with just a few of the lines. They do not wash off. They are only skin deep (when I cut into the beans, you can't see anything wrong on the inside). Does anyone know what these are?

What varieties are they
There are a few varieties such as rattlesnake and cascade giant that have grayiish streaks that eventialy turn purple
Charlie

I get occasional lines on my snaps that look somewhat like faint scars. I don't know for sure but I have speculated that they actually are scars. If the wind is strong, the occasional pod will get knocked against something that causes a little damage, but because snaps grow so fast, the damage will keep happening to a slightly different spot almost minute by minute, so you end up with a line of lightly damaged tissue.

That's my guess. They don't grow so fast that I stick around to watch the beans grow LOL.

I was told it was from the soil being nutrient deficiant The best remedy for this is compost

material and manure A good indication of nutrient deficiancy is if your beans are short and curled instead of long and straight Good Luck