One of the tube on my preamp flashes on turn on.

I am new in the tube world. I recently bought a tube preamp along with two Mullard 12AT7 and everything was fine until today when one tube flashes for a second on turn on and after that it is ok. Is it anything wrong with the tube?
There are two kinds of tubes, those that do that and those that don't.

In other words, don't worry about it. Mullards are excellent and the most often tube mentioned when discussing turn on "flash."

If it really drives you crazy, Telefunken's don't "flash" and they too are excellent, sometimes better, depending on the circuit.
Some old tubes do this, there's a technical term for it which I'm forgetting. If you check out the Upscale Audio site you'll see a description of it there, I believe they specifically mention certain Mullard tubes as being prone to flashing. I had a Bugle Boy 12AU7 that did the same thing, but sounded great. Nothing to worry about.
100% normal for most any tubes from the Philips Europe factories. It is just momentary resistance in the heater.

So it will happen on Philips Miniwatt, Mullard, Amperex, Radiotechnique, Valvo, and Adzam 12AT7 12AU7 12AX7 and others.