Metformin vs Metformin HCI

I wanted to know if there is any difference between the two. What does the HCI stand for?

Ms.

I want to know if there is a difference also.  My daugter went to Diabetic Camp this summer and we did not bring her Metformin ER 750 mg pills.  The doctor there ordered some for the week.  They were Metformin HCL ER 750 mg pills.  She was constantly falling low throughout the whole week.  She is on a very high dosage of insulin (over 100 units per day) and taking the pill, which didn't seem to help.  But the new pills (which were a oval brick red in color), were working very, very well.  Her insulin was down to below 50 units per day.  Now that we have gone back to the old pills, she runs high all the time. 

 Is there a difference?  I am going to ask the doctor to put her on these pills.