What’s wrong with my Christmas Cactus?

I think it’s overwatering (which could explain the brown colour of the base) but I’m not sure what the white section is caused by?

I water it once a week by leaving it in water for like 10-20 minutes- should I do it every other week instead? I used to only occasionally water it by pouring some water into it whenever I remembered it needed it

It gets enough light (but is a bit crowded and cramped into a windowsill sharing with other smaller plants)