Yes the invasive sea lamprey will target walleyes if given the chance. They target the bottom dwellers more often because they are easier to catch and attach themselves to. Notably lake trout and small mouths as they spend a large portion of their time on the bottom. Lampreys' were a contributing factor to the decline of lake trout in the great lakes. They prefer fish that are notably more bloody than other species but are opportunistic in what is available. They suck the fishes body fluid out for consumption and can and do cause the death of the host fish. So the theory it is a lamprey scar is a very good possibility.