That looks pretty bad. You need to remove the outer and also the inner part for a thorough inspection and possible replacement. And replace the bolts and nuts.
The outer part is easy to remove (you have to secure the rig with halyards and remove the shrouds, one side at a time - mast removal not necessary, just makes it easier). Then the inner part can slide down the rod. But to remove entirely, it's a lot more work.
For sealing ours I used butyl, because it's great with never leaking. But because butyl is messy, I then put a bevelled edge around with white MS polymer sealant, which protects the butyl from collecting dirt.
Don't leave this unattended. We met a German on a 38 Ocean where this exact corrosion has caused the shroud chainplate (or bolts) to fail and the mast to fall over the side. I re-checked ours after that and replaced the bolts, as one had a small crack in the head.