From what I gather precipitation seems to be a common occurrence with Seachem iron. The form of iron they use is susceptible to oxidizing but readily available to plants. If the KH is high or you use buffering agents this can happen. If this is the case you may want to consider changing to a stronger chelator instead of gluconate such as EDTA. This should prevent the precipitation if it's caused by elevated KH or other buffering agents. As homemadepopcorn said dosing phosphate at the same time can cause precipitation as well.