DexCool, and it's never been flushed/cleaned/replaced...serviced.
You can try one of the typical flush solutions, and you're probably gonna havta
do it three or more times.....IF you manage to get all the slime out.
If you can't get it 'clean' where you can see,
you can imagine what's left in the block, heater core, etc.
Time for a professional opinion / inspection / leak test
to see if there's been any damage to the manifold gaskets
and any leaking in that area....