Something white in cage corner?

[TsukiCatHammy]

Hi, so this morning I looked in at my hammy to notice, and this is a terrible explanation, something that almost looked like white paint up the plastic of the corner where he goes to the bathroom...

I feel like ive seen this before and I can't recall if it was with my current hamster or my dwarf.

I read somewhere that it just happens if they pee on plastic... is this true... because its weird and I can't figure it out