Very most likely it’s zinc oxide. Heat plus water plus an (alcalic I guess?) detergent accelerates the process. The bolts have probably started oxidizing before you cleaned them and the residue is part of that, now visible after the dirt came off.

Depending on what result you want, you could go ahead and just use the bolts and conserve the look with wax spray or you could remove all the zinc and oxides by sandblasting, wirebrushing or a rock tumbler. Have them zinc plated by a company that does galvanic treatment, it’s usually surprisingly cheap