Looks like you have, or had, a leak to me. When was it last removed and cleaned properly, seals replaced etc. I can't see it that clearly on my phone but I suspect its just cosmetic. You need to remove the stack and have a proper look
Hi I cleaned heat exchanger and replaced the anode in 2019 (boat was in yard all of last year). Yeah I'll take it out clean it soon and hope there isn't much damage, but brace myself for an expensive part.
If you do find any pitting on the sealing surfaces you should be able to fix this with chemical metal provided its not too bad but hopefully it just needs a good clean up and a bit of paint
I also have the older design with the single bolt on the header. But with mine, it's hard to see the "O" ring when it's full assembled. It tucks away in that cleverly designed 3 point contact. Yours on the other hand is clearly visible, so perhaps worth checking the other header. I assume you know that reassembly needs some care to keep the tubestack "floating" rather than hard against one end.
I have to change my anode twice per season, but estuarine water is hard on it.