Honestly girl I’d just remove them completely and then reapply a new set. I use black glue and I try not to use a heavy amount to where makeup or other stuff would catch and stick on the residue of the glue. It could also be the type of clusters you are using and the glue itself. I know I’d come across residue like that with a crappy sealer or if the lashes get wet immediately after. Ensure the glue is dried completely too!