I am 31 weeks pregnant. For the past day I have had decreased fetal movement water-like diarrhia. Should I call my midwife or is it normal?

Call your midwife. If you are concerned about decreased movement of the baby, call your OB or midwife directly. Better dosage than sorry. .
Yes. Any decrease in fetal movement warrants a consult to your provider. Diarrhea is not normal but could be just regular run of the mill viral diarrhea nit related to the fetal movement.