I will be honest.
Melzalgald's regeneration should be High-Mid. Its regeneration is
outdated since mid-2018; the page had Low-High regeneration since its
creation. The marble seems to be just
solid not a liquid.
Old Low-High:
'''Low-High:''' ''Able to regenerate from a puddle/drop of blood, or a small piece of your body.''
New Low-High:
'''Low-High:''' The ability to regenerate from having no solid parts of the body remaining. This can range from a puddle or drop of blood to even a single cell. For robots and machines, this can also include regenerating from a liquid state.
In Boros's case, I could see his regeneration being possibly be compared to
Melzalgald's updated regeneration.