Most barnacles are not parasites, except a couple of unique species (Sacculina sp. parasatise crabs, where they can actually castrate male crabs and ‘feminise’ them, making them unable to reproduce).
The barnacles seen encrusting on this horseshoe crab are more typical (sessile filter feeders), and are partaking in what is called a commensal relationship – that is, the host is not harmed or benefited from the barnacle, but the barnacle does gain a benefit by having a motile surface to settle on that may help it move to new feeding grounds or escape predators. Some barnacles may even provide additional armour or camouflage to the host.
Parasites are organisms that harm the host in order to gain a benefit, and there are many forms of parasitism. One could argue that a heavily encrusted host such as this horseshoe crab could be harmed. This many barnacles may increase drag in the water, prevent the horseshoe crab from sensing its surroundings, damage the carapace, or make the horseshoe crab more noticeable to predators.