If the red cross appears when charging and then goes away when the charging is complete , it's just part of the phones system process and should be ignored because you can't make it go away while charging.
If the red cross appears when charging and the battery is not charging then that red cross is there to tell you that there is something wrong with the battery and it's not charging.
On my phone when charging a small lightning bolt appears on the battery symbol to signify charging also a tiny led lights up orange and turns to green when the battery is fully charged.
There are a couple of options that you can do if you chose to.
1. It's possible that the red x should not be there and you should get a different one and the red x is a way for the phone to tell you that there is something wrong with the battery , even though it's charging. Have you looked at the owners manual for the phone ? It will tell you the process of the battery charging and it will tell you what the screen should look like and if the symbol should be a red x or something else.
2. Buy a new battery and see if it does the same thing , I know that's costing money but if both are good then you have a spare battery and if not then you have your solution if the new battery gets rid of the red x.
3.If you don't want to buy a new battery then try going to the cell store and asking them , they can tell you right away if it should be there or not.