Do not use stainless bolts. It is an aluminum casting and the SS bolts in al form a more reactive galvanic cell if the threads get wet. The SS bolts won't corrode, but the expensive al casting will. Just use the same alloy bolts and coat threads with grease.
The white stuff is corrosion product from the al corroding. If there is contact with the bronze cap and the joint is damp, you have an active galvanic cell and the al will get pitted.
No fan of al housing heat exchangers for this reason.
Periodic disassembly, cleaning, resealing and greasing tends to slow the corrosion.
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