I'm impressed with how well you recreated the famicase at-style. Also does a great job of giving the feeling that you're really caving. Unfortunately though I ended up getting frustrated trying to hunt the elusive red mushrooms and so ended up using cheats to reach the end.

Quick thoughts

• It'd be interesting the the mushrooms could be located via additional cues, rather than just line of sight and the compass. Like unique audio, or a trail of spores floating through the air.
• More traversal options would make exploring more satisfying,  for example a grapple (unlocked by finding it or paying mushrooms) or a mushroom that launches you into the air when you jump on it (jump pad). Even the debug teleport would work, give it a limited range and cost a mushroom to use.
• Compass: Really needs an above/below indicator and it would be helpful if it showed landmarks, like the doors.
• Would have appreciated navigation aids, like trail markers. Perhaps you could even allow the player to place them.
  
• At times I felt it was too hard too see.  I wonder if a LIDAR-like effect with a cooldown would help.