Click on each part of the 8x8 square of cubes in order to mine each piece. The gray ones have to be clicked to be turned into colored ones, in which they can be mined out.
The white score represents the remaining number of layers to clear. The person who clears the last layer will get some kind of reward from me. The green number represents the number of points that you currently have. They will be used in a shop later on that will add things such as a "Clear Layer" and upgraded pickaxes, each having durability. That will come in a future update.