Dracaena compactas growing mud?!

Hi all, I'm new to growing plants and I got dracaena compactas recently. They don't look the best as they've been neglected but I'm bringing them back! Curious about the mud/soil growing on the leaves and settle in the centre. Its not from the environment as this was normal for the ones at my work. Does anyone know what it is? Is it some type of mold or secretion that hardens? Thank you!

Also, these photos are after I've wiped them best I can and they keep coming back!