Leaking from the freeze plugs. Those indentions in the block are freeze plugs,
Sand cores are used to form the internal cavities when the engine block or cylinder head(s) is cast. These cavities are usually the coolant passages. Holes are designed into the casting to support internal sand forms, and to facilitate the removal of the sand after the casting has cooled. Core plugs are usually thin metal cups press fitted into the casting holes, but may be made of rubber or other materials. In some high-performance engines the core plugs are large diameter cast metal threaded pipe plugs.
Core plugs can often be a source of leaks due to corrosion caused by cooling system water. Although modern antifreeze chemicals do not evaporate and may be considered "permanent", anti-corrosion additives gradually deplete and must be replenished. Failure to do this periodic maintenance accelerates corrosion of engine parts, and the thin metal core plugs are often the first components to start leaking.
Difficulty or ease of core plug replacement depends upon physical accessibility in a crowded engine compartment. In many cases the plug area will be difficult to reach, and using a mallet to perform maintenance or replacement will be nearly impossible without special facilities for partial or complete removal of the engine. Specialized copper or rubber replacement plugs are available which can be expanded by using a wrench when access is a problem, though engine removal may still be required in some cases.
The term freeze plug is slang, the correct name of the press-in plugs is core plug. It is mistakenly thought that the purpose of these plugs is to be pushed out and save the block from cracking if the engine has water in it and it happens to freeze. This is nothing more than an urban legend.
The purpose of the plugs is to fill the holes that were made during the casting process, so the foundry could remove the core sand from the coolant passages. Saving the block from cracking in case of a freeze was never the manufacturer's intent for these plugs.
I think the only way to fix it is remove it and put a new one.
Sometimes you can get by cleaning it good, but I do not recommend it.
I think usually machine shops have to remove them and install them properly.