I'm surprised to hear that such apps exist. I had no clue.
I'm not sure they are such a good idea beyond the absolute beginner stage, though. The ability to hear whether your fretting is clean and correct is a skill that must be developed. There are a few suggestions in this thread about ways to do this (play slow, play single notes). Eventually, you'll just feel it, but it takes a good long while. In the mean time, this skill must be worked regularly. To the suggestion of how to do this I would add recording yourself and listening back. A video would allow you to hear and see what you are doing.