You are looking at the inside of a Falcon 900 center tank that's been contaminated.
This microbial growth (all the little black/brown spots in the pictures) when left untreated will cause damage to the aircraft structure by eventually causing corrosion.
It will also attack everything else in the tanks...like plumbing, wire connections, pumps, injectors, clamps, hoses, everything!
These tanks were cleaned, resealed and saved from expensive intensive corrosion repairs just in time!