Moldy particle board is not safe, whether the mold continues to grow or not. The spores can cause an allergic reaction, the particle board has been weakened and cannot be relied upon to perform its structural function, and the mold may continue to grow if the air is humid. Get rid of it.
Plywood is the best choice between these two. Particle board can swell if water gets to it. You would be better off if you use a tile backer board. Concrete based or a fiber type. They are made for this application.
A towel gets wetter and wetter as it dries.
Yes, the stain typically gets darker as it dries.
a towel
Towel
the atmosphere.
A towel
a towel
A towel.
TOWEL
Towels
A towel.