Thank you for asking. Since your mulch has broken down in a relatively short timeframe, it is likely made up of some organic product like corn starch. It may still contain some petroleum-based glue or binding substances to make it into a mulch. A photo- or biodegradable mulch should still break down completely to water and carbon dioxide. There could be some intermediate breakdown products that would potentially be harmful. Even if that is the case, these should attach to soil particles or decomposing organic materials in the upper soil layer until they have also broken down. It is unlikely harmful substances from the mulch would leach into the soil and to the potatoes. The potatoes you are growing should still be safe to harvest and eat.