Acid catalyst decarboxylation of 1-carboxyl-tryptoline
Your suggested mechanism should be acceptable (need two more steps, protonation and deprotonation, see last two steps of my scheme below), if you have isolated the carboxylic acid product (compound $\bf{1}$ in my scheme). If compound $\bf{1}$ is not isolable, I suggest following path would be taken to decarboxylation:

Your reaction is an analog to Pictet Spengler reaction (Wikipedia). When the intermediate $\bf{I}$ is formed during the reaction as depicted in the mechanism, 1,2-hydride transfer is possible to give more stable carbocation $\bf{II}$, which is resonance stabilized $(\bf{\ce{II &lt;-&gt; III}})$. The intermediate $\bf{III}$ can easily undergo decarboxylation to give some what stable enamine product, $\bf{2}$. In acidic medium, this enamine $\bf{2}$ would undergo protonation to give the intermediate $\bf{IV}$, which would then deprotonated to gain aromaticity and give thermodynamically stable indole product, $\bf{3}$ (the intended product).