Glass slags are non-metallic inclusions which remain undissolved in the matrix, and the crystalline slags can dissolve in the crystalline structure by forming defects in the crystal lattice of the metals
There are many differences between glassy and crystalline mold slag. The first is that glassy mold slag has stronger radiation heat transfer than crystalline mold slag. The second is that crystalline slag layer is more rough that glassy slag and easy to form air gap between mold and slag layer. The third is that glass formation of slag is beneficial to lubrication between mould and steel because the viscosity of slag is not critically increased during glass formation
For a discussion about amourphous or poorly crystallized Mg(OH)2; in slags: https://www.researchgate.net/post/Is_MgOH2_a_crystalline_or_an_amorphous_phase