Baked on cement splashes in a large quantity I wouldn’t touch for a customer. The only way is to give the glass a good soak with soapy water, and use a Stanley blade to remove, then rinse with the jets, not using the brush. Once the loose stuff is off , then wash with the W FP brush. I did this for a customer last week, but he is also a mate, so wouldn’t make a fuss if the glass got marked. Particularly when the splashes were caused by his own mixer.
I wouldn’t do it for a customer for fear of being made to pay for a new pane if the glass got scratched. Definitely make the builder remove them.