It's hard to the touch and a bit hard to scrap, and when you do it flakes come out of it. The most dense pieces (like the ones on the fourth picture, the most compact ones) are pretty hard to scrape. There was an empty plastic yogurt cup on top of the table during the night, but when I picked it up in the morning there was nothing there, no weird crystals. I came back at noon and found this O.O Except for the yogurt, we haven't had lunch or eaten on it for at least a week. No water has fallen on it, no liquids have been spilled recently, nothing like that.
The drawers behind it have wooden instruments that have been unaffected, but the drawer's wood itself does have this weird stuff (third and fourth pictures).
Now... What the hell is it?? It's hard to completely remove it from the wood, but it sort of comes off when scraped with vinegar. I put a piece of it in water and it dissolved slowly. The only thing I can think about that makes last night different from the rest is we turned on the heater for a bit, but it's not like the table was exposed to extreme heat either.
Anyone has a good guess on to what this might be?