Yes, the Samsung PRO Plus 256GB SD is compatible with the Sony FX30.
Samsung PRO Plus is a UHS-I U3 (V30-class) card. The FX30's own Help Guide states a U3/V30 SDXC card works at the 200 Mbps recording bit-rate; the camera's 4K modes (280Mbps+) need V60/V90, which this card does not meet.
Limitation
Cannot record the higher-bitrate XAVC HS/S/S-I 4K modes (280Mbps+) -- they require a V60 or V90 card, this card is U3/V30 per the FX30's own Help Guide. Works for XAVC S HD and the 200 Mbps recording modes.
“When the recording bit-rate is 200 Mbps, you can also record using an SDXC card (U3/V30).”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)