I'd take a photo of the side of your home and then call the landlady.
"Hi, Susan, this is Jpcher, I own the house next to your rental unit. Yesterday I was outside and I noticed that the side of my house and the windows closest to your unit was covered in grime and dust after the work you had done. If you'll be having those workers return to finish up would you ask them to clean it up? If not, do you want me to contract someone to do it and just send you the invoice for reimbursement?"
I like this, although I might try to stress by some alternate wording that this isn't something that a wet wipe in a couple of places is going to fix. Maybe "....that the entire side of my house and windows facing your unit were completely covered in a thick layer of grime and dust after the work you had done. It's enough that there's no way even a good rainstorm would wash it off sufficiently. (Insert remainder of script above.) And also, when you have work done like this in the future, could you give us a heads up ahead of time? We had two windows open during the work, and afterward, we noticed that we had a sizable about of dust and grime INSIDE the house, necessitating a lot of extra washing and cleaning. If we'd known the extent of the mess we'd be facing, we could have closed the windows and prevented that."