  This paper deals with the following fractional Choquard equation
$$\varepsilon^{2s}(-\Delta)^su
+Vu=\varepsilon^{-\alpha}(I_\alpha*|u|^p)|u|^{p-2}u\ \ \ \mathrm{in}\
\mathbb{R}^N,$$ where $\varepsilon>0$ is a small parameter, $(-\Delta)^s$ is
the fractional Laplacian, $N>2s$, $s\in(0,1)$, $\alpha\in\big((N-4s)_{+},
N\big)$, $p\in[2, \frac{N+\alpha}{N-2s})$, $I_\alpha$ is a Riesz potential,
$V\in C\big(\mathbb{R}^N, [0, +\infty)\big)$ is an electric potential. Under
some assumptions on the decay rate of $V$ and the corresponding range of $p$,
we prove that the problem has a family of solutions $\{u_\varepsilon\}$
concentrating at a local minimum of $V$ as $\varepsilon\to 0$. Since the
potential $V$ decays at infinity, we need to employ a type of penalized
argument and implement delicate analysis on the both nonlocal terms to
establish regularity, positivity and asymptotic behaviour of $u_\varepsilon$,
which is totally different from the local case. As a contrast, we also develop
some nonexistence results, which imply that the assumptions on $V$ and $p$ for
the existence of $u_\varepsilon$ are almost optimal. To prove our main results,
a general strong maximum principle and comparison function for the weak
solutions of fractional Laplacian equations are established. The main methods
in this paper are variational methods, penalized technique and some comparison
principle developed in this paper.
