Aquascaper
Fish Aficionado
Slate is a dark gray, flat, natural stone and you'll find it in any building supplier.
It's just a fungus that feeds on nutrients leaching from the wood. One the nutrients have been used up, it'll go away on it's own.
It isn't harmful to fish, although I know it looks unslightly.
For the record; don't boil wood; it cooks the fibres and makes it release even more tannins/nutrients in the long term, as well as causing it to rot faster.
