Most eggcrate is made of thermoplastic. It doesn't have any phosphates directly in it if it's polystyrene (which the white eggcrate usually is pure polystyrene). Make sure you wash it really really good to get any residue off that may have been left on during manufacturing. There was an article I read once on some forum (NY Reefers? Maybe NJ?) that some guys in a chemical engineering lab actually tested the stuff and they couldn't force it to leach phosphates unless it came in direct contact with a kalk solution, and even then you're talking minute traces ( something like 0.01% by volume or some nonsense leeching out over the first 40 days).
You might get algae growing on it but that doesn't mean it's from it leeching anything. You might have enough nutrients in your system combined with a nice clean plastic eggcrate that currently has no other life on it competing for the space plus stray light from a fuge or something which could result in it growing there. Under those conditions it'd grow if you put a piece of glass there or anything else.
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