puchfinnland

What you have going on there is Galvanic corrosion.

we had a simmilar case on a half year old bike.

the owner just left his custom made frame bike out in the weather all summer,
the bottle bosses are steel rivnuts, the little plastic cable guide under the BB has a stainless screw holding it in,

you will need to strip the bike down completly,
locate all the places alu and steel come in contact.
have it glass beaded, and then proper aluminum primer and a paint job.
grease every steel screw and component before putting it back together.

notes
headset are prone to cause trouble also
are the pop rivits on the cable guides stainless or steel?
the cable housing ends? plastic or shiny steel? replace with plastic shimano type
the bottlebosses-steel rivnuts?
was the BB greased before it was threaded in?
a drainhole in the frame bottom is a wise idea before repainting.
grease every screw and adjuster.

you could even put plastic tape between the front deraileur and the frame to help