Often that is elementary sulfur that forms when the thiosulfate decomposes. You might want to figure out why it happens. In your case, "end of quantity" will be a good enough explanation, even more so if you are not using the stuff at regular rate (i.e. within weeks of opening), but have a half used canister about for months.
The severity of the issue depends on the formulation of the fixer. In some it is a sign of fatal damage, in others it does not affect the action as long as the solid is out of harm's way - in old hanger systems there often is a solid block of caked sulfur at the bottom of the fixer tank, to no harmful effect.
If it occurs in paper fixer used for temporary, non-critical purposes, you might filter or decant the bottle to get rid of the precipitate and test whether it still is fixing at the expected rate - small amounts will not visibly affect the prints, and archival issues will only strike after months or years.
For film use or exhibition grade prints you'd better dump the stuff - you positively don't want to have anything solid embed in a film emulsion, nor risk having a valuable print go blotchy, discolour or fade out.
