sacrificial anode

Perhaps? Are you using a 24 volt system with two (2) batteries only, or a 36 volt with only three (3) batteries? If so then perhaps your ground for the troller is not on the cranking battery thus you have a connection of the positive and ground on the boat system discharging when the motor is deployed, through the natural mineral content acting as a contact between the posts in the water. Otherwise it could be some other item such as the trim motor having an issue and discharging through the water in a similar fashion, hydraulic plates or any other powered item can cause this if there is a positive discharge exposed through the water, as the ground is always in contact with the water through the engine as a frame ground source.

Since the trolling batteries are isolated, then perhaps there is an engine component putting power in the water. If water in your area is high in calcium then some build up is normal, though not if it is eating at components. Check the anode or trim tab plate for corrosion from electrolysis.

Long distance there is little we can do to assist Stampers, though determining if the trollingnmotor use increases the electrolysis is a step. The items to check are those where power contacts the water. Electrolysis like you seem to havebis very rare and very hard to find.

Are you getting a lot of prop slippage?The prop is not getting hot and boiling the water ?Are there small pits on the prop?I would think the inside of the prop would look the same as the outside. Thick white chalk everywhere.The thick aluminum oxide chalk should come off with cleaner/polish. If you have to scrape it off it is like burned on.Is there any other aluminum on the motor that is oxidizing?

That is only the propeller it appears. Unusual and we do see that in clear reservoirs. Though it is not usually as bad as yours, the side anodes do not look to be deteriorating, check the one under the trim pump.