Because the base form is Fermi, there is more than one meaning to malferma. It can mean opening as in la malferma parolado de kongreso, or it can mean open (not restricted) as in Malferma Tago ĉe la centra oficejo de UEA.

Questerlej, you will come across the idea that Esperanto roots have a specific grammatical class (eg verb noun adjective), and the grammarians have erected a convoluted edifice based on this idea.

But in practice Esperanto is a bit more pragmatic than the strict application of this idea would suggest. This, incidentally, is demonstrated by the actual usage of malferma, where the verbal idea can be very weak, if present at all.

PS. A quick check with the Tekstaro shows that malferma is much more common than aperta. It is possible that aperta was introduced to deal with cases of being open where there was no act of opening.

However it looks as though the Esperantists felt that this was not necessary and the meaning of malferma could be comfortably extended and its interpretation be left to a sort of common sense and not totally bound to the supposed verbal character of the root.

If I see a sign outside a shop Malfermaj Horoj 9.30 - 16.00, I am not going to suppose that the shop keeper is constantly opening and reopening the shop for six and a half hours. I am going to think that the horo de malfermo is 9.30 and the shop then remains malfermita until 1600