most likely it's biofilm - i.e., it's thick coatings of bacteria. Generally speaking they're harmless and also unavoidable unless you take steps to eliminate bacteria from your system (which would also destroy your tank).
Do you dose any sort of liquid carbon source? i.e., the prodibio system, zeofood, vodka, sugar, vinegar, etc.? An relative increase in system wide biofilms in places like piping is a direct result of dosing liquid carbon, but they will happen no matter what you do. If the system is old enough for the biofilms to have reached critical mass they are very likely just getting dislodged by the pump turning bcd on.
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