Well, a problem with the Dragonborn DLC. After I left Apocrypha on that mission to go to ask Storn for the secrets of the Skaal, I can't fast travel or wait. It says I can't do so when I'm taking health damage. I am NOT taking health damage. As well, I glow green when I swim.

And no, I did not accept Neloth's test.

PLEASE help me!

Welcome to the forums.

You say you glow green when you swim. That could mean that you actually are taking health damage. Maybe you have been poisoned? Have you checked your active effects display? That will tell you if you are currently poisoned or infected by disease or whatever.

This is a problem that sometimes happens when entering Apocrypha after reading one of the black books (most commonly either "The Sallow Regent" or "Filament and Filagree"). It can usually be solved by re-reading the last "black book" that you read (meaning the one that caused the issue to begin with). This will transport you back to Apocrypha. Find a spot in Apocrypha where you aren't taking health damage (if possible) anymore and read the book one more time (which should transport you back to Solstheim).

Note: It may necessary to redo this process more than once if the issue persists.

For me it was the Winds of Change book. The strangest thing solved it; I went back in, and stood in one of those pools of that weird liquid, and I started glowing green like I did when I swam, then I read the book again and went back to Solstheim. I go to my map and click on an icon, and it says I can fast travel again.

That's strange, but it's good you figured it out. Maybe it was originally caused by stepping in one of those black pools then, they do deplete your health.