There are several reasons this could be happening:
- Flukes
- External parasite
- Something in the water is bothering it (water quality, stray current, etc.)
If you do frequent water changes and it's in clean, well-filtered water, you can probably eliminate a water quality problem as long as all parameters are in check (low nitrate, zero nitrite/ammonia, pH hasn't crashed). Most of the time when fish rub their faces against decorations or substrate it's probably gill flukes.