Looks like it has the color of iron oxide in it. Which may come from exposed steel parts in the damp environment of the pump reservoir. I've seen similar slime develop in past condensate pumps.

Last time one of mine died I bought this because it was the only one I could get on short notice. Found it on Amazon with delivery the next day, I did not find a pump locally but I'll admit I didn't search every plumbing supply house in the area.

This pump automatically self-cleans (which you can initiate manually if you want) among it's many other features. It will sense when it has not been run in a while and perform a clean cycle. There is no float switch to beak or jam. It has an audible alarm for malfunctions. And many other features.

I normally keep a spare on the shelf so if one craps out I have a spare on hand and I'm ready to deal with it 24/7. When I bought this I was looking for one to replace my spare that I was going to put in service then when I found this I said leave that Utilitech pump on the shelf, I'm going with the Diversitech pump now. The form factor is identical to the Utiltech pump. The hanger lugs, connections, etc are all the same so an easy swap out.

I am happy with my choice as it has been performing flawlessly for almost a year now. I will buy another soon as it is time to replace the other condensate pump which is getting noisy and on its last legs.

https://www.diversitech.com/en-US/item/condensate-management/condensate-pumps/clearvue%E2%84%A2-pump--IQP-120

Looks like it has the color of iron oxide in it. Which may come from exposed steel parts in the damp environment of the pump reservoir. I've seen similar slime develop in past condensate pumps.

Thanks for the pump info Bob...from the ClearVue brochure:

"ClearVue even senses when water has remained stagnant in the tank for extended periods of time and will run a complete tank drain to reduce buildup and slime growth in the tank."

It sounds like that's a common occurrence so I'll be checking the 2 other condensation pumps.

That ClearVue product seems pretty nice and it isn't that much more expensive than the Little Giant pumps I installed. I'm going to order one and try it out.   

The tank drain option would be especially nice for the Honeywell dehumidifier as it only runs during the heating season. Shut down the dehumidifier and push the tank drain button...done.

Now I need to go look at mine. 
I have only had this furnace for 2 winters and everything has been great. This is the first one I have had that requires a condensate pump.