some kind of small organisms - problematic?

I have noticed an increasing number of small, unsophisticated organisms in the tank. the are about 3mm long at most, about 1.5mm wide, and relatively transparent with a reddish tinge. They are also relatively thin and cling to shells, tank walls, etc. They do not appear to have any predators in the tank (i.e., the crabs, shrimps and fish do not bother them).

Are these anything to worry about (and if so, any suggestions other than vacuuming them)? I replace 20% of the water each 2 weeks, and add ReefBuilder (Seachem), Reef Advantage Calcium (Seachem) and Coral Vite (Kent Marine) each week. I have re-homed a number of my previous fish and am now down to a long-nosed hawkfish and a member of the basslet family (purple front/gold rear with a black circular dot on its dorsal - forgot the name), two shrimp (1 parasite eater) and a variety of hermit crabs.

Thanks in advance.