Fenris - It is more of a "snake like thing" than a "worm like thing" but whatever. If you approach and make a melee attack and then retreat it will get an attack of opportunity. You could use cunning action to avoid that, but you are already using it (and thus your bonus action) to hide. I just want to be clear on the rule implications.
Uthal - Nobody on the parties side of the rock fall (meaning all but Fenris) were hurt by the rocks or covered with anything but dust. So you quickly find Gerdar right in front of you. Fenris is on the other side of whatever mess the rock fall made. I just want to be really clear about any actions people are taking here so that I am not assuming anything. In front of you the tunnel has some rubble. You don't know the extent of the blockage because of the dust. It may be passable or it may not - you will have to find out. Doing so, or trying to cross, would mean entering the rock fall area.(Assuming passage is possible, of course.)
If people want to move into the rockfall area to find out how bad it is blocked and if passage is possible please be clear about the following:
1. What you will do if it is blocked.
2. What you will do if it is only partially blocked.
3. If you move forward into the area are you taking any precautions at all? If so, what?
4. How fast are you trying to move? (linked to #3 above.)
5. Anything else you can think of.
Right now dust fills the air so it is hard to see much.