I guess you are talking about Euphorbia graminea 'Inneuphdia' (which goes by the trade name Diamond Frost). Planters can definitely lead to moisture problems (both excessive moisture and drought conditions). Have you checked the moisture level deep down in the pot? The soil may appear dry on top and still be soaking wet deeper down in the medium. What have you done to assure good drainage, and what type of planting medium are you using? How deep is your pot?

I had the same problem with my 'Diamond Frost' here on the east coast of FL after a week of pouring rain. All of a sudden, my sidewalk lined beautiful stand of Euphorbia turned yellow and lost their leaves almost overnight. My plants were in ground. I'm wondering if I should cut them back or just leave them alone to recover. It is the beginning of hurricane season here. Will I lose them with more rain? Should I just pull them and wait until next summer to plant again?