Does terminology for changes in membrane potential refer to additive or multiplicative change?
Note to others: this question appears to be about terminology rather than the biophysics behind the membrane potential.

When the terms "increase" and "decrease" are used relative to membrane potential, they refer to additive changes in potential, not to multiplicative change or effects on absolute value. The standard is to relate the potential inside to the potential outside of a cell (so a negative potential means the inside is more negative than the outside; potentials in biology are always relative, just like in physics).

Therefore, a membrane potential decrease means that the value of the potential becomes more negative, and a membrane potential increase means the value of the potential becomes more positive. The changes on the absolute value of the potential depend only on where you start from.

Other terms that are more clear to use are hyperpolarization and depolarization, where hyperpolarization always refers to the membrane potential becoming more negative than rest, and depolarization refers to the membrane potential becoming more positive than rest. Repolarization is also used, particularly in the context of action potentials, to refer to voltage changes in the direction of hyperpolarization when the cell is currently depolarized.