It will be pva ( glue ), which you put on before rendering to help render stick. What a pig to get off, explain to customer and put your price in to remove
Darren
Seriously, tell them straight.
"The builders haven't covered up the windows, and the adhesive used for the rendering has been (i take its sprayed?) over the windows. This needs to be specially removed and takes a lot of time.
I can do this, but it will cost a lot more than your usual window cleaning does. Im sorry its the nature of the works involved! Personally id get the builders to pay for it since they didn't do a good job protecting them!"