Without a closer look it is hard to tell, but I get a strong impression that the central "ice" pellet is not in fact pure ice, but something salty or otherwise water-soluble that inhibits freezing of water. It might be actual sodium chloride grains, or some other partly soluble mineral or soap or the like. This material then apparently diffused outwards from the centre. The highest concentration occurred round the pellet, where it completely prevented frost (or caused it to melt before teh picture was taken) and lower concentrations further out caused reduced frost formation.

Of course, it could also be a waxy or greasy material that discouraged formation of frost crystals without actually preventing freezing.

Sorry, but that is the best I can do without more information.

Mooi loop,

Jon