Is this bacteria buildup on my rock wool cause for concern? + more

Howdy all, second time growing, first time posting here.
I recently made the transition to bring my seedlings from my germination light box into the tent. During that transition I placed the rock wool cube into hydroton (which had been washed prior) and several days later I began to see this buildup of black specs around the rock wool. If it is cause for concern, is it too late to transfer the seedling from the rock wool to soil? Any advice on this front?

Additional dilemma: Somehow, I placed two seeds into the same rock wool cube, and one sprouted shortly after the other did. What is the best way to preserve both seedlings and their roots? Can I safely plant them in soil instead? Please advise on the best way to go about this.

Also, can rock wool be placed into soil. If so, will the seedling grow as it would in the hydroton?
Please advise!

What size is rockwool cube?
I occasionally move plants started in rockwool into soil with no problems.
Most recent was a Chongs Choice clone in a 4" cube.
I cut off about half the rockwool, cutting the corners off, avoiding the roots as much as possible.
Place in soil and cover rockwool on top so cube is slightly covered by the soil.

Your 2 seeds in the cube, how close are they together? Is it a small cube?
Any pictures?
You might be able to split them with a sharp blade but can't say for sure without a picture.