It's dried up fuel mixture. Tank was probably full when it was put away.
Acetone should work. Never tried fresh gas, but that might work, too.
If there's pinholes, I agree with everyone else. Once the tanks clean, you'll find them.
The funnest part is gonna be flipping the bike over and getting all that crap out. If it was me, I'd take it completely apart so you're just dealing with the frame, otherwise you'll never get it fully clean.
And if there are pinholes, use Caswell tank sealer. You'll definitely need it all apart so you can roll the frame around to completely coat the tank