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Get that plate off there, its salt . Wood was placed in salt for the curing process , to dry faster without checking. Just wipe it off ,and light sand , it will continue to come out till gone. Then apply a true oil finish.
The wood was probably finished or wiped down with a natural oil. Some molds just love these oils as food and if the conditions are right, warm and damp, reproduce accordingly.
My guess is you won't get that plate off easy, The screws will be rusted , and fused to the wood, just try and remove it. I may be wrong , but can't hurt in checking
If the problem is confined to the bottom of the stock as the picture suggests I think you have a moisture problem coming from under your vault. Does it sit on cement? Do you have a vapor barrier between your vault and the cement? I have my storage on raised wooden platforms that are vented out the sides and a heavy plastic vapor between the bases and the cement and another vapor barrier between the wooden base and the vault. Excess moisture, warmth and tung oil or BLO on the stocks will give you just what you see.
For basement storage I should have made clear why I have my storage on a platform raised off the cement. The cement floor is usually quite cool compared to the ambient air in the room. Result would be the safe metal above the ground is warmer than the base of the safe which sets things up for low level condensation to form in the safe. I raise mine several inches on a wooden base and ventilate the base sides to let air the same temperature as the room to get in as well as to allow any moisture to vent out.