Infection in my barrel aged stout?

[JSS89]

Hi there!

I have have had an 8% stout in an oak barrel for just over 2 months. I was planning to bottle it tomorrow but saw today a white skinny lager on top of the beer. I have made some tests showing that FG and pH has not changed. Normally an infection will affect either one of the or both. What do you guys think? Is it infected? Taste and smells just. anyone experienced anything like this?

Thanks!