Manganese(II) hydroxide is not only a strong base but is also amphoteric as it dissolves in strong alkali solution to form hydroxomanganates:
From the solutions of $\ce{Mn(OH)2}$ in 50% Sodium hydroxide solution,
the hydroxometalates $\ce{Na2[Mn(OH)4)}]$ is isolated.
But before forming hydroxomangantes, intermediate ions like $\ce{MnO2^2-}$ and $\ce{HMnO2-}$ are formed. This paper(*) discuss this fact:
The high solubility of manganese hydroxide in acid solution and its
low solubility in alkali solutions give a qualitative indication of
the fact that
manganese hydroxide is a strong base. This seems to be further verified by the observation that manganese salts do not hydrolyze
appreciably. The complete verification of this prediction is given in
the subsequent calculation of the acidic and basic constants of
manganese hydroxide.
The following equations probably indicate the reaction that manganese
hydroxide undergoes in water, in acid, and in alkali.
(...)
$$\ce{Mn(OH)2(s) + OH- -> HMnO2- + H2O }$$
$$\ce{Mn(OH)2(s) + 2OH- ->MnO2^2- + 2H2O}$$
(...)
The Equilibrium in Alkaline Solutions.
If manganese hydroxide is a strong base then we may expect its
hydrogen ion dissociation to be very slight, particularly the second
dissociation. Unfortunately, the solubility is so slight in the range
where it is possible to calculate these acid dissociation values that
no data were obtained.
(...)
$$\ce{Mn(OH)2 -> H+ + HMnO2- K = 1 \times 10^{-19}}$$
*http://pubs.acs.org/doi/pdf/10.1021/ja01852a001