You are likely not going to have any problems being in the Colorado climate if you leave that diffuser where it is...but having a diffuser in the deepest part o the pond can be an issue so it may be a good idea to just drag the diffuser up by the airline until it is at a depth of about 1/2 your maximum depth....in this case keeping the diffuser at 7 feet would be fine.
The reason for this is that keeping the diffuser in the deep end would eliminate a thermocline, which is the stratification of the pond, where warmer waters sit at the bottom. If your diffuser is in the deep section it will continuously be pushing the war water towards the surface where it will cool off at the surface and return below in a cycle that will start to "super cool" your water which can cause harm to fish.
It is rare that this will happen in a big pond even if you are pushing 5.2 CFM in a single diffuser because even this will circulate a whole lot of water it won't have the same impact as in a smaller pond. By a big pond I mean something over a fifth of an acre.
To prevent fish kills in small ponds a small air bubbler will work to keep a hole in the ice as would a sump pump pumping water towards the surface. In larger ponds the larger deicer system works well at keeping a large hole open in the ice which is sometimes desired for duck hunters.
Of course...the risk an danger of over-chilling your pond is remote and this is a "worst-case" scenario and likely wouldn't happen in your pond with the species you are keeping but it wouldn't hurt to just haul the diffuser to a shallower place.
Your pond may have stayed cool due to the placement of the diffuser and the capacity of your aeration/mixing system. Normally an aeration system should circulate the entire volume of the pond on a fairly rapid schedule...in small ponds we try and circulate the volume of the pond every few hours...in larger ponds it isn't always possible...but an efficient air stone diffuser can circulate several thousand gallons per minute so that sort of mixing is completely possible.
The deeper your diffuser is the more you will be able to warm up the water temperature according to the ambient air temperature. In mid summer if your air temperature is 80 degrees F then you should be able to raise the pond water temperature within 5 degrees of that with the proper movement of cold water towards the surface.
Any slimy algae like that should be treated with natural bacteria. I use dry or liquids depending on the season...it's a cheap and effective way to knock down the algae! Copper based products can have their place but in most ponds any sort of harsh copper product can create a viscous cycle where you need to add more and more of the product to keep the algae away.
Using a natural bacteria is more of a long-term strategy...the bacteria eats nutrients that cause algae and improves depth and reduces muck. Using sch bacteria will not have a dramatic killing effect on algae but over time it will create a more balanced Eco-system so I have found it works best in a long-term strategy of the ponds I deal with.
You may want to look at an end of season "shock treatment" of natural bacteria. There are some extremely potent liquid bacterial products for ponds that help consume organic material in the Autumn and they even work under the ice. Normally we do a treatment near the end of October and a follow-up two weeks later at which point we leave the pond alone until spring.
I hope this helps!
Let me know by email if you ever have any other questions regarding the care and maintenance of your pond in the Summer.