What is the name of this digital low pass filter?
I'm new, so I can't add this comment to Matt L.'s answer.

It is not an exponential filter, the equation is actually:

$$ y[n] \ = \  \alpha \, x[n] \ + \ ( 1 - \alpha ) \, x[n-1] $$

So it is a very short FIR filter, not an IIR filter.  I'm not expert enough to know a specific name.

Ced

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Followup:

I want to thank everyone for their upvotes.  Yes, I've contributed to comp.dsp and I am a blogger at dsprelated.com.

Like all of you, I suspect that the intent of the function was to be an exponential filter and was coded erroneously.

If I were to name the filter as coded, I would call it a "Linear Interpolation Filter", or perhaps a "Subsamplesize Time Shift filter" when $\alpha$ is between zero and one.  It would make more sense as such if the $\alpha$ and $(1-\alpha)$ coefficients were reversed.