I see what you are saying, but both times you added fresh surface area to your tank in the form of egg crate. Bacteria, algae and hopefully corals are all competing for living space in our tanks. Adding a piece of egg crate is like building a condo in a neighborhood. Someone needs to move in and colonize it. It was colonized by turf algae in your case as it was able to do so faster than anything else. Just from that evidence, I can't blame the material itself as it could have been anything. I have gotten the same effect from adding a piece of rock to a tank that has been sitting out of water for a while. If I add it to a lit part of the tank, it will be colonized by whatever the quickest spreading algae is in the tank. If I add it to the sump, then later to the tank... no algae problems with it because it is not colonized by surface bacteria and all sorts of other things that inhibit the rapid growth of algae. It happens to my DIY frag disks as well.
Inherently egg crate is a great place for algae to grow. Ask waste water folk and they use it all the time. In our frag tanks we put it under light and in good flow. ++ The egg crate provides a good surface for something to grow on as it has a slight texture and lots of surface area. + Unlike liverock, algae eating critters can't live inside the egg crate so competition is lower then on LR. + Snails don't like being on it very much. + Hermit crabs have a hard time crossing it. + Algae eating fish can't access all the places the algae can hide, plus many peoples frag tanks are to small for tangs etc. ++ Frag tanks are often set up very quickly without the cycle processes we put our displays through. I can't blame it for attracting algae, but I can't see facts that support styrene leeching chemicals that promote algae. At least not yet. If I can get my hands on acrylic egg crate, I will experiment with it.
Now if I sound like an egg crate salesman, I can have 10 boxes of egg crate delivered to your house every week for just two easy payments of 49.99 plus s&h.