What's this canoe treated with?

It is difficult to say from pictures alone but my guess would be excess canvas filler. If your canoe was hand made by someone who was not familiar with building canoes like this then they may have applied so much filler that it leaked through the planks like that. Many old filler formulas contained large amounts of white lead so you may need to be careful not to eat or breathe any while removing it.

It could also be a canoe that was immersed in the ocean frequently so this may just be salt that leached out from the water. Good luck,

Benson