Weird Mushroom Growths

[Avalon]

Hello all,

I've been growing edible closed-cup mushrooms from one of those kits and am having problems. I've got pinhead mushrooms at the moment which is good, but at around the same time as they started developing I also got one big, white, non-mushroomy lump growing in each box. Please see attached picture.

What on earth are they? I'm not even going to try to eat them, but the instruction pack doesn't say anything about them and I don't seem able to find any information about them online.

I'm not growing aliens, am I?