heat exchanger freeze plug leaking

[Mjresq4]

While doing a post-trip bilge check I noticed a slight smell of antifreeze and that the overflow reservoir on my starboard engine was a little low. Further inspection found a slight leak at the heat exchanger freeze plug. It looks like 4 each engine and only 1 of the 8 are leaking. So, I have a few questions. What caused this to happen, regular wear and tear or is there a bigger problem looming? Is this a simple DIY repair as it looks to be? During the trip, both engines ran between 172-174 as they normally do.