If the mass of the bob of simple pendulum decreases, its time period will increase/decrease ??

for a simple pendulum,

T = 2π root(l/g) where l is the length of the pendulum .

thus the time period of a simple pendulum is independent of the mass of the bob.

thus if the mass of the bob decreases or increases....the time period of its motion will not change as the length of the string will remain same.

kindly approve the above reply if it is the answer to your problem.

Time Period of Simple Pendulum is independent of The mass of the bob !