103.4b

If an effect allows a player to perform an action "any time [that player] could mulligan," the player may perform that action at a time they would declare whether they will take a mulligan. This need not be in the first round of mulligans. Other players may have already made their mulligan declarations by the time the player has the option to perform this action. If the player performs the action, they then declare whether they will take a mulligan.

Serum Powder.You can use Serum Powder’s second ability only while it’s in your hand. If this card is in your hand, you can choose either to mulligan or to use Serum Powder’s ability. Using the ability doesn’t prevent you from taking further mulligans, and taking a mulligan doesn’t prevent you from using a Serum Powder’s ability if you happen to draw one in your new hand.