Is this your only euphyllia? Was it happy before? Depending on those answers...
Flow:
Make sure it's not too much. Tentacles should just have a slight sway back and forth. People say moderate flow but I find it is more low flow. I designed my rock work and flow around where my euphyllia collection would be. (Sheltered).
Alk:
Make sure it's not too low or high. My LPS love 10dkh
Mood:
It could just be moody. One of my hammers did this for 2 days last week.