Hi Mia,
this is not possible to write Music on an app while playing the harp.
At least not under normal circumstances.
This requires a MIDI harp. Each string with a pickup and a converter for translating the string vibration in to a machine language (MIDI)
Witt MID data a software can recognize and understand what the Notation should be.
Think about how should a machine (App) identify which individual notes are played when you for example Play a cluster with 6 tones.
For MIDI-Harp exist two very different ways
Option A: each string just triggers the converter
Option B: In 2006 was my official presentation of the world new MIDI HARP with the advanced technology of frequency recognition …. while the converter detects the exact string vibration and translates the frequency to MIDI data.
http://harfenbau.de/midi-harp.html
I hope his gives you some explantation.
Sorry for the language…
best regards