What is this? planted some tomato seeds and this has popped up
Whatever it is I wouldn't worry about it. Just pull it out.
looks like some sort of fungus/mushroom it was probably in the compost and the warmth/wet of the propagator has kicked into life, just pull it out and compost it.
I did think it looked a little too moist too - might open one end up a little for air
I watched War of the worlds on TV a short time ago ,do you think one might have survived,looks familiar,,,,
I have the same propagator as you
Use fine seed compost and make sure it comes to the top of the trays otherwise the seedlings will struggle to get above the sides to the light.