Slimy inside of green onions
Yes?

That is, I'm not entirely sure what your question is... but if you're asking if they are, than yes, insides of green onions can be slimy.

Since you're seeing the problem in in-season, fresh, local, big green onions, I suspect the slime you're seeing is simply - sap.  The green onions may be fresh enough to still contain living sap, and large enough to keep their moisture.  Out of season and non-local, shipping and storage then you're probably not getting onions with quite so much life in them.  If this is the case, I would expect the sliminess to also correlate with really bright, fresh flavor.

If your question is what can you do about it, if you don't want the sliminess?  You might let the onions sit in your fridge for a few days - that should give the plant time to dry out a bit.  You might buy slightly older, drier onions.  you might chop them to size, then let dry (or rinse and then let dry) before using in other preparations.  If the issue is the freshness of the sap, there's nothing else to be done.