My acoustic neuroma has come back. is it possible that it could be cancer this time?

Unlikely: Did you have radiation therapy? Acoustic neuroma can come back with longer follow up, although short term local control rates are very high (98% at 5 yrs). If they do enlarge, most often they remain benign.

Rarely: If you had surgical removal and tumor persists or recurs, there is almost no chance it would be malignant. If it has grown after treatment with radiation, there is approximately a 1:1000 chance it could have malignant transformation.