Using a mandoline, slice the zucchini lengthwise into long, thin strips. Lay the strips in a single layer on a tray lined with a kitchen towel or a paper towel. Sprinkle salt over the zucchini strips and set aside. (Adding salt draws out the moisture and makes the strips pliable.)

Drain the crab of any excess moisture, then combine with egg white in a bowl. Season with salt and pepper. Place a spoonful of crab mixture on one end of each zucchini slice. Roll up, starting with end that has crab mixture. Place rolls, seam sides down, on a flat surface. Place flour on a large shallow plate; season with salt and pepper. Place panko on large shallow plate; season with salt and pepper. Place beaten eggs in a shallow bowl. Coat the fritters in seasoned flour, shaking off the excess. Dip fritters in egg, and then in seasoned panko, coating completely. Place on a baking tray and set aside in the refrigerator for 15 minutes to let the coating set.

Pour 2 to 3 inches oil into a large heavy pot and heat to 350 degrees F. Fry a few rolls at a time for about 4 minutes, until crumbs are golden. Remove rolls from oil; drain on paper towels.

For the Grapefruit Aioli, in a small saucepan simmer grapefruit juice and zest until reduced by half. Remove from heat and cool. Combine mayonnaise, sour cream, and cooled grapefruit reduction in a blender. Blend until combined. Serve with warm fritters.