Thanks for checking it out, and glad you enjoyed!
I'm thinking about adding a bag menu in the corner of the screen. I want to make it open up and show you collected items with a small blurb. I think an animation of items floating to and from it would look nice and help with clarity. As for the specific instances you mentioned: the gray octagon was supposed to display the pieces you've collected of the yin yang symbol (and can be used after picking up the first one), and the rock (spikes) in the cave get destroyed by the boulder you picked up in the mining cave!