Why does magnesium have an exceptionally low melting point?
Following up on my comment, even though I'm not convinced that this is a 'real' answer to the question.  Consulting the SGTE database of elemental free energies [1] (with various updates over the years), one can obtain the melting temperatures for the hcp, fcc, and bcc phases of beryllium, magnesium, calcium, strontium, and barium (rough guesses looking at plots — consider good to approx. $\pm\pu{5 K}):$

$$
\begin{array}{lrrrcc}
\hline
\text{Element} &amp; T_\mathrm{m}(\text{hcp})/\pu{K} &amp; T_\mathrm{m}(\text{bcc})/\pu{K} &amp; T_\mathrm{m}(\text{fcc})/\pu{K} &amp; \text{Phase at RT} &amp; \text{Melt phase} \\ 
\hline
\ce{Be} &amp; 1544 &amp; 1560 &amp; 1018 &amp; \text{hcp} &amp; \text{bcc} \\
\ce{Mg} &amp; 922 &amp; 755 &amp; 705 &amp; \text{hcp} &amp; \text{hcp} \\
\ce{Ca} &amp; 940 &amp; 1114 &amp; 1063 &amp; \text{fcc} &amp; \text{bcc} \\
\ce{Sr} &amp; 900 &amp; 1050 &amp; 1030 &amp; \text{fcc} &amp; \text{bcc} \\
\ce{Ba} &amp; 635 &amp; 1000 &amp; 709 &amp; \text{bcc} &amp; \text{bcc}\\
\hline
\end{array}
$$

Several things to note, amongst them the variety of which crystal structure is most stable for the different elements at different temperatures. Also, the hcp phase's melting point does almost(!) monotonically decrease going down the table.  The biggest anomaly would actually seem to be the relative instability of the bcc and fcc phases of magnesium (it is the only one that remains hcp up to the melting point).

Overall, it would appear that the group 2 elements display a wide variety of preferences for crystal structure, and hence bonding energy configurations.  Is $\ce{Mg}$ unusual? Not really…

References


Dinsdale, A. T. SGTE Data for Pure Elements. Calphad 1991, 15 (4), 317–425. DOI: 10.1016/0364-5916(91)90030-N.