Looks like a common shroom we have here in the UK that only grows on birch, as it is in your viv - Piptoporus betulinus. It starts out all white, and as it matures turns brown on top. When mature it has a texture similar to polystyrene, and in the UK was once used for sharpening razor blades, hence it's common name of 'razor strop fungus'. They do get huge hope your viv is BIG!
Those are cool. I have my first mushrooms appearing right now in one of my vivs! They are growing a little bit every day. They are not the shelfing type though. I'll post picks When they are big enough to photograph (I have no photography skills).
Anyhow, how fast are yours growing?
Is it indeed growing on birch?
What sort of microclimate is it growing in, in your viv (light, humidity, temp, etc)?