Help! Betta keeps on flaring

Hi, my betta keeps on flaring at the spot behind my HOB filter. He started doing that since I made the water level higher. Any idea why he keeps flaring at that particular spot? He seems to do it more when the aquarium light is on. I don’t know if this helps, but I took out the filter and saw some whitish stuff growing inside with the usual algae. I just cleaned it out. My filet had been off for almost 6 hours now…
Photo of white stuff in propeller of filter. Sorry for bad image quality.

 

He probably sees his reflection on the glass in that spot

 

The white stuff is probably just regular build up...I have that, too, sometimes.

I stuck a big Indian almond leaf between the glass and my filter. You could try that, and see if it helps (to get him to stop flaring right there if it bothers you).

 

The white stuff is probably just regular build up...I have that, too, sometimes.

I stuck a big Indian almond leaf between the glass and my filter. You could try that, and see if it helps (to get him to stop flaring right there if it bothers you).

I don’t have almond leaves right now, I’m waiting for them to be shipped. I’m just scared he will stress himself out.

 

You can try putting cardboard or paper behind the tank in that spot so he can't see his reflection

 

You can try putting cardboard or paper behind the tank in that spot so he can't see his reflection

It worked! Thanks!