Why? You can only run MQL on the desktop, so why code it on a phone or tablet?
If you really must, there is already many apps for text editing on iOS that recognise the C/C++ syntax which can easily be used to write MQL code.
You could even use the browser on your tablet, on the GitHub site which already supports MQL4/5 editing.
Here is an example of a Code Editor that works on multiple
types of devices, including iOS and allows user configuration of file
types and context coloring (just define one yourself for MQL):
Did you read the documentation on the site?
Did you create an account for yourself on the website first?
Plus, I am not their Customer Support nor do I use the product. I simply used my head and Googled to find iOS editors, something you could have done yourself before posting!