I'm going to guess that it is or was probably something from inside the muffler burning . ( ie: rust particles,grass,mouse nest,etc. ) When was the last time it was running before this happened,had it been sitting a while?
Sometimes pieces of internal rusted exhaust get heated up and break loose (real smal pieces) and they come out like that. This is why some small engine exhaust systems have a "spark arresting muffler." Some of your state or federal forest services require this on vehicles (ATV, Tractors etc.) that are operated in parks.
They do it during the daytime, but you just don't see it. Back in the day, I had to grind corn with our M Farmall. When pulling hard, once the governor topped out she would kind of "pop" and blow a shower of sparks. Unnerving inside a barn with hay around! Our corn crib was built in the end corner of the barn.
after running my test engine 2a016-3 for 18 hr. straight at 3600 rpms turning a not running briggs model 19d with a restrictor plate over the intake for load testing, in the late evening the header pipes off the heads were glowing orange up to the bends in the pipes, and was shooting small blue with yellow tip flames out the header collector pipe, no carbon in there.
sometimes i run peters john deere model b in the evening and see sparks from the muffler when pulling a heavy load, sometimes it's good to run a engine under a good load to clean it out,