After letting the XionLab Rust Converter dry, a dull white residue appeared on the surface. Is it normal?

What you are seeing is some residual salts from the phosphating process. This happens occasionally on very rusted and porous surfaces, it is a byproduct of the rust being converted.  In most situations, this can be simply wiped off before applying a top coat. If you use a mild detergent to aid in this process, make sure to rinse thoroughly and let it fully dry before applying the top coat. If you are not applying a top coat, this residue will slough off on its own.