Identifying stains on joists/pirlings in loft

[Michaelsf90]

Recently gone up to put Christmas decorations back and noticed these sandy coloured stairs on the joists and purlings in the loft. House is 1930s so seen its fair share of dust and dirt. Just wondering if anyone has any idea what it is. If I scrape it with my fingernail it seems to come off. Or is it literally just some kind of dust or build up with the age of the house

 

[noseall]

Looks like a nice healthy dry roof. Probably old torching snot stains from the original roof, prior to the felt and re-roof.

 

[Michaelsf90]

The roof is still doing a brilliant job after 80 odd years. The felt is knackered and perishing tho and we occasionally suffer from condensation despite having roof tile vents in. I'm gonna have to get it re roofed in the next 5 years or so. I wondered if it was some kind of mould