If it was ferrous oxalate instead, you can do a potassium ferricyanide (part B of a cyanotype kit) test - add a drop of an acidified (may not be required since you are already adding oxalic acid) solution of it to a bit of this slurry, if it results in a blue color (that of Prussian blue) then either you have ferrous oxalate by mistake from the supplier or you have ferric oxalate that is partly converted to ferrous.
I recently ordered Ferric Oxalate from a supplier in Mumbai and it looks similar and behaves very much the same like yours. Just want to check if you found any solution for dissolving the stuff and make it work for your purposes?
It's a pity that my naive DIY Ferric Oxalate worked well for Kallitype and the lab synthesised one is a dud.