Protoss Counter To Midgame Mutalisk
This article discusses possible counters to the Zerg Midgame Mutalisks.
Mutalisks are known to harass mineral lines and kill workers, and this is also true for ZvP.
In order to save Probes, if you scout Mutalisks or know that your opponent is going Mutalisks, try to throw down 2 or 3 Photon Cannons on each mineral line. Don't forget though that a good number of Mutalisks can easily destroy Photon Cannons, so extra support is required.
If it's possible to pump out many Corsairs, this will be a hard counter to the Mutalisks; however, a good Zerg player will usually bring Scourge along and attempt to kill the Corsairs instantly.
Archons are also very effective against Mutalisks. If you have 2-3 Photon Cannons plus an Archon, there is not much Mutalisks can do to touch your Probes.
Another strong counter is to simply counter the Zerg with what you have. For example, if you went with a +1 Zealot timing push with speed, and his Mutalisks are coming to your base, you should try to counter his third or natural. Your Zealots are going to do a lot more damage than his Mutalisks, and he is going to be low on ground units regardless. He will be forced to pull his Mutalisks back or lose his bases, and during this time you can get some Archons made and some Photon Cannons laid down.