Yes, residue from the spray.
In future, just don't foliar spray at all, it is really inefficient and more likely to cause problms rather than solve them.
Applying nutrients via the roots is all that is needed and far more effective than any sort of foliar feeding.
The people in the grow shop just want you to buy stuff, they work on the theory of selling more products when things go wrong.
If you are absolutely determined to use the spray fertilizers, apply them just before lights out and never use them when the plants are flowering.