Cliff,
Hard to say with the information you have given. There are several polymers that could fit that description and probably thousands of chemical combinations used to make those polymers. I have a lot of plastics reference materials and can look it up for you if you give me more information, but without a manufacturer or more specific information it is hard to say. Even then, the specific additives in this foam rubber can vary from manufacturer to manufacturer and batch to batch.
Much of what is known about polymer toxicity is highly technical and still being researched by many organizations including the EPA and FDA. All polymers are permeable to a certain degree and most are somewhat subject to oxidation, what is released into your tank due to hydrolitic breakdown, microbial attack, or other types of degradation is guesswork. The tricky thing about using these chemicals is that you may never notice the adverse ill effects as they may be subtle, mutagenic, etc, and could manifest itself years later in ways you might not imagine (e.g., low fertility). Some of this information is coming out about bisphenol A (a common comonent of most epoxies) and alligators (there's a thread around here somewhere) and recently the US joined the EU and others in phasing out PBDE in polyurethane foams (like those used in car seats, mattresses, pillows, etc) because of toxicity concerns.
I wouldn't use a synthetic foam in direct contact with water, but YMMV...
Marcos