What is this room with a smiling spider in it?
This is an item in the middle of the non-destructible stones. More specifically, it's the "Spider Bite".
Spider Bite - Adds a chance to shoot slowing tears, reducing speed of enemy movement and any projectile fired by them.
The star on the map means that you can pick up the item. If you gain flight, you can fly over the stones and pick it up regularly.
The room is a Normal Room in which can rarely spawn an item.