ACR X AUTHORIZED XING OF ACTIVE RWY HAD CONFLICT WITH SMA Y ON TKOF. EVASIVE ACTION TAKEN BY SMA Y. SYS ERROR.

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I GOT TAXI CLRNC TO LEAVE GATE AT PHL AND 'TAXI TO 27L VIA TAXIWAYS ALPHA AND CHARLIE HOLD SHORT RWY 27R.' APCHING RWY 27R ON CHARLIE; GND CLRED US TO 'CROSS RWYS 27R -- TWR WILL ISSUE SEQUENCE ON OTHER SIDE.' PRIOR TO XING I NOTICED 2 COMMUTER TYPE LTTS XING RWY 27R UPFIELD PROBABLY ON TAXIWAY U AS WELL AS A COMMERCIAL JET ON FINAL FOR RWY 27R; NONE OF WHICH SEEMED TO BE A POTENTIAL CONFLICT WITH US. AS WE WERE XING 27R; I NOTICED A SMALL SINGLE ENG SMA Y COMING TOWARDS US ON THE GND ON RWY 27R. AT FIRST I THOUGHT HE WAS ON LNDG ROLLOUT OR TAXIING DOWN THE RWY (BASED ON HIS SLOW SPD). THEN HE ROTATED AND BANKED SHARPLY TO THE R PRESUMABLY TO MISS US. FACTORS -- WHEN CLRED TO CROSS; ALL I SAW ON THE RWY WERE 2 COMMUTER ACFT XING (POSSIBLY ON TAXIWAY U). THE SMA Y DEPARTING RWY 27R WAS GRAY OR BLACK AND BECAUSE OF HIS SMALL SIZE AND COLOR; BLENDED RIGHT IN WITH THE RWY ENVIRONMENT. HE DISPLAYED NO TAXI OR LNDG LIGHT WHICH WOULD HAVE MADE HIM MORE VISIBLE. GND CTLR WAS VERY BUSY BUT I COMMENTED TO FO HOW QUICKLY WE WERE GETTING TO THE ACTIVE (PRIOR TO XING RWY 27R). THERE SEEMED TO BE 2 GND CTLR POS IN US AT PHL AT THAT TIME; 121.65 AND 121.9. WHAT I BELIEVE HAPPENED WAS THAT TWR TOLD GND TO CROSS THOSE GUYS (MEANING COMMUTERS ON U) AND GND CROSSED US ALSO RIGHT INTO THE PATH OF THE SMA Y WHO HAD BEEN CLRED FOR TKOF. USE OF AN EXTRA CTLR POS FOR XING RWYS (LIKE WHAT IS USED IN BOS) MIGHT HAVE BEEN HELPFUL AS WELL AS SPECIFIC COORD BTWN CTLRS. ALSO IT WOULD BE EASIER TO SEE SMALLER ACFT IF THEY USE THEIR LNDG AND TAXI LIGHTS JUST LIKE THE LARGER ACFT -- ESPECIALLY AT LARGER ARPTS.