I was looking around my world when a random completely white block appeared. It is larger than a full block - similar to a minecart in size but fully cuboid. It can be pushed by pistons and has gravity but I could walk into it and when looking at it, nothing could be placed or destroyed through it. Not sure if it was gravel being strange as it was in a mountain.

It can also be pushed into the nether like mobs can which i have now done which leads me to suspect it is an entity glitch?