I was shown a picture of a window yesterday from a fb group. On the outside of the glazing there was  tracking or streak marks running the whole way down the pane. Didn't know this was a "thing" but there were several other posters who have encountered the same occurrence. Couple of the folks maintain this is lime burn(moisture running down before plaster has set), as we are going down the wet dash route (including lime) I got to thinking, if the mix contacts the window frame (alu clad) is there a chance of reaction? Also would I be better using white sand/cement rather than lime? One or two people had to replace glass as the marks would not come off, spending a good few quid on windows and this to happen would be a nightmare.

Yes anything like that could mark the alu cladding. Ours said to make sure we kept the cladding free from any run off from the building materials. Ours weren’t so bad because it was just the run off from the wood cladding treatment so I just had a soft cloth and clean water on standby for a few weeks.