Santeria / Voodoo Question

I own a rental property and the tenants are Haitian. I am having the exterior painted and when I was reviewing the details of the paint job with the painter, we noticed that there was salt or a similar powdery substance on the windowsills in the front of the house.

Anyone know what this is? I can ask the tenants directly, but I also don't want to make any assumptions "just because" they are Haitian.

It's going to come off when the house is pressure washed, so if they want to remove it themselves for a religious reason, I want to give them a head's up.