Jumping to a conclusion...it is possible that what you have is some old stock and now comprises a mixture of Fe3+ basic sulfate, Fe3+ oxides/hydroxides plus the remains of the original Fe2+ sulfate. Atmospheric oxygen is capable of oxidising Fe2+ to Fe3+. The kinetics are affected by pH amongst other things but overall it is a recognised fact that ferrous sulfate exposed to air can form a mixture of various ferrous and ferric compounds.
Assuming the original compound was indeed Fe2+ sulfate the extent to which it has reacted is not possible to tell without chemical analysis. Although the pure Fe3+ ion is a pale lilac colour, in its compounds generally yellow/orange/browns predominate. But colour alone does not give a reliable indication of the extent of the chemical change.
I have no idea of how this might affect its efficacy as a garden treatment.