What will be the effect on equilibrium price of ice cream if there is decrease in price of milk?

Dear Student,
The equilibrium price is the price at which demands equals supply. It is the price at which there are neither unsatisfied buyers nor unsatisfied sellers. The equilibrium price can not change unless demand or supply schedules change.
When there is a fall in the price of other commodities , market demand will either increase or decrease depending upon whether other commodities are substitutes or compliments .
Case I) If other commodities are substitutes:  market demand for the commodity in relation to price of the substitute good will decrease.
Case II) If other commodities are compliments  : it will increase the demand for the commodity in relation to price of the complementary good. 

If milk and ice cream are complementary goods, this means they are consumed jointly. In such a case, the cross price effect moves in a way that an increase in the price of one good will decrease the demand for the other and vice versa.

Now, the effect of fall in price of milk on its own equilibrium price would be that, as the price falls below the equilibrium point, there is an excess demand for the good. Too many buyers tend the suppliers to increase the prices to gain more profit due to shortage and price increases till it returns back to its equilibrium where quantity is also in equilibrium.
Regards