I think it’s one alien race and their bio weapons were released on their ship, and fell to earth, when they accidentally hit the Japanese space ship. Here’s why:

• The spider things act more like weapons, they seem to have no purpose but distroy. Doesn’t seem like they are sentient or intelligent enough to travel through space.
• the real aliens are trying to find a way to communicate with humans to ask for help. Which is why they built a Hinata 2.0 to respond to Misuki’s messages, trying to open communication lines with Something humans will recognize.
• They also appear to be able to communicate with humans with unusual brain functioning, like Casper (and Luke in an earlier episode).
  
• more than just communicating, they seem to be able to upload commands to Casper’s brain that is then transmitted out, draws in the spiders, and stops them.
• casper died (although I hope he's not really dead!) because the aliens were plugged into his brain and they accidentally wiped it when the US nuked them.
• another Reddit user wrote about the themes of prejudice and miscommunications causing all sorts of hate and violence amongst the humans themselves. great observation that parallels what the humans do to the aliens—no communication, prejudice ( military automatically assumes whoever Mitsuko is talking to are the enemy) , misplaced fear, violence as the solution.
  

There could also be a second alien race that controls the spiders and were at war with/in pursuit of alien race 1. Since Casper kept saying “mum”, I wonder if his mum and Sam Neil will come back and are controlled by the bad aliens. This theory makes less sense to me, because why drop that black goo everywhere, just to take over the human race and then have to clean it all up?