white patches on felt in loft

DontDoItYourself

I hope someone can help me with this puzzle. I had my loft extra re-insulated quite a number of years ago, and I'm fairly certain that the airflow has been OK up there since. But, during the latter half of last year we have had these patches appear (never before!). At first, the patches appeared to look like frost. I then thought maybe it is down to condensation, but looking again, it gives the impression of what looks like crystalisation (if that makes sense). I haven't as yet tried to see if it wipes off. I have provided an image to help demonstrate this. It only seems to have appeared across the front side of the house. My house (terraced) was built in the 1950's. Is it possible for felt just to become "too old" and starts to break down? Any help and guidance would really  be appreciated. thanks