  The $RE$Al(Si,Ge) ($RE$ = rare earth) family, known to break both the
inversion- and time-reversal symmetries, represents one of the most suitable
platforms for investigating the interplay between correlated-electron phenomena
and topologically nontrivial bands. Here, we report on systematic magnetic,
transport, and muon-spin rotation and relaxation ($\mu$SR) measurements on
(Nd,Sm)AlGe single crystals, which exhibit antiferromagnetic (AFM) transitions
at $T_\mathrm{N} = 6.1$ and 5.9 K, respectively. In addition, NdAlGe undergoes
also an incommensurate-to-commensurate ferrimagnetic transition at 4.5 K. Weak
transverse-field $\mu$SR measurements confirm the AFM transitions, featuring a
$\sim$90 % magnetic volume fraction. In both cases, zero-field (ZF) $\mu$SR
measurements reveal a more disordered internal field distribution in NdAlGe
than in SmAlGe, reflected in a larger transverse muon-spin relaxation rate
$\lambda^\mathrm{T}$ at $T \ll T_\mathrm{N}$. This may be due to the complex
magnetic structure of NdAlGe, which undergoes a series of metamagnetic
transitions in an external magnetic field, while SmAlGe shows only a robust AFM
order. In NdAlGe, the topological Hall effect (THE) appears between the first
and the second metamagnetic transitions for $H \parallel c$, while it is absent
in SmAlGe. Such THE in NdAlGe is most likely attributed to the field-induced
topological spin textures. The longitudinal muon-spin relaxation rate
$\lambda^\mathrm{L}(T)$, diverges near the AFM order, followed by a clear drop
at $T < T_\mathrm{N}$. In the magnetically ordered state, spin fluctuations are
significantly stronger in NdAlGe than in SmAlGe. In general, our
longitudinal-field $\mu$SR data indicate vigorous spin fluctuations in NdAlGe,
thus providing valuable insights into the origin of THE and of the possible
topological spin textures in $RE$Al(Si,Ge) Weyl semimetals.
